Enable Minitest/AssertNil cop to address DEPRECATED: Use assert_nil if expecting nil warning

This commit addresses the `DEPRECATED: Use assert_nil if expecting nil` warning at
https://buildkite.com/rails/rails/builds/116902#01953bd2-7999-468a-bcd6-5e9b68964fa6/1270-1276
by enabling `Minitest/AssertNil` available since RuboCop Minitest 0.1.0.

```ruby
$ cd railties
$ RAILS_STRICT_WARNINGS=1 bin/test test/application/configuration_test.rb:4031
Run options: --seed 1261

DEPRECATED: Use assert_nil if expecting nil from /home/yahonda/src/github.com/rails/rails/railties/test/application/configuration_test.rb:4031. This will fail in Minitest 6.
E

Error:
ApplicationTests::ConfigurationTest#test_ActiveStorage.variant_processor_uses_mini_magick_without_Rails_7_defaults:
ActiveSupport::RaiseWarnings::WarningError: DEPRECATED: Use assert_nil if expecting nil from /home/yahonda/src/github.com/rails/rails/railties/test/application/configuration_test.rb:4031. This will fail in Minitest 6.

    /home/yahonda/src/github.com/rails/rails/activesupport/lib/active_support/testing/strict_warnings.rb:38:in 'ActiveSupport::RaiseWarnings#warn'

bin/test test/application/configuration_test.rb:4026

Finished in 1.568723s, 0.6375 runs/s, 0.6375 assertions/s.
1 runs, 1 assertions, 0 failures, 1 errors, 0 skips
$
```

Here are the output of rubocop -a that corrects other assertions that are not deprecated.

```ruby
$ bundle exec rubocop -a
... snip ...
Offenses:

actionpack/test/controller/integration_test.rb:498:7: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(request.parameters["leaks"]).
      assert_predicate request.parameters["leaks"], :nil?
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
actiontext/test/unit/model_test.rb:21:7: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(message.content).
      assert_predicate message.content, :nil?
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
actiontext/test/unit/model_test.rb:149:7: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(message.content).
      assert_predicate message.content, :nil?
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
activemodel/test/cases/type/boolean_test.rb:10:9: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(type.cast("")).
        assert_predicate type.cast(""), :nil?
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
activemodel/test/cases/type/boolean_test.rb:11:9: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(type.cast(nil)).
        assert_predicate type.cast(nil), :nil?
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
activemodel/test/cases/validations_test.rb:255:5: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(t.author_name).
    assert_predicate t.author_name, :nil?
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
activerecord/test/cases/associations/has_many_through_associations_test.rb:1246:5: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(post[:author_count]).
    assert_predicate post[:author_count], :nil?
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
railties/test/application/configuration_test.rb:4031:7: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(ActiveStorage.variant_processor).
      assert_equal nil, ActiveStorage.variant_processor
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

3345 files inspected, 8 offenses detected, 8 offenses corrected
$
```

Refer to
https://github.com/rubocop/rubocop-minitest/blob/master/CHANGELOG.md#010-2019-09-01
https://github.com/minitest/minitest/blob/master/History.rdoc#5230--2024-05-15-
minitest/minitest@f0f17b99

Author: yahonda

.rubocop.yml

@@ -382,6 +382,9 @@ Performance/RedundantStringChars:
 Performance/StringInclude:
   Enabled: true
 
+Minitest/AssertNil:
+  Enabled: true
+
 Minitest/AssertPredicate:
   Enabled: true
 

actionpack/test/controller/integration_test.rb

@@ -495,7 +495,7 @@ def test_post_then_get_with_parameters_do_not_leak_across_requests
       assert_equal "foo=bar", request.env["QUERY_STRING"]
       assert_equal "foo=bar", request.query_string
       assert_equal "bar", request.parameters["foo"]
-      assert_predicate request.parameters["leaks"], :nil?
+      assert_nil request.parameters["leaks"]
     end
   end
 

actiontext/test/unit/model_test.rb

@@ -18,7 +18,7 @@ class ActionText::ModelTest < ActiveSupport::TestCase
   test "without content" do
     assert_difference("ActionText::RichText.count" => 0) do
       message = Message.create!(subject: "Greetings")
-      assert_predicate message.content, :nil?
+      assert_nil message.content
       assert_predicate message.content, :blank?
       assert_predicate message.content, :empty?
       assert_not message.content?
@@ -146,7 +146,7 @@ class ActionText::ModelTest < ActiveSupport::TestCase
   test "with blank content and store_if_blank: false" do
     assert_difference("ActionText::RichText.count" => 0) do
       message = MessageWithoutBlanks.create!(subject: "Greetings", content: "")
-      assert_predicate message.content, :nil?
+      assert_nil message.content
       assert_predicate message.content, :blank?
       assert_predicate message.content, :empty?
       assert_not message.content?

activemodel/test/cases/type/boolean_test.rb

@@ -7,8 +7,8 @@ module Type
     class BooleanTest < ActiveModel::TestCase
       def test_type_cast_boolean
         type = Type::Boolean.new
-        assert_predicate type.cast(""), :nil?
-        assert_predicate type.cast(nil), :nil?
+        assert_nil type.cast("")
+        assert_nil type.cast(nil)
 
         assert type.cast(true)
         assert type.cast(1)

activemodel/test/cases/validations_test.rb

@@ -252,7 +252,7 @@ def test_validation_with_if_and_on
 
     # If block should not fire
     assert_predicate t, :valid?
-    assert_predicate t.author_name, :nil?
+    assert_nil t.author_name
 
     # If block should fire
     assert t.invalid?(:update)

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -1243,7 +1243,7 @@ def test_deleting_from_has_many_through_a_belongs_to_should_not_try_to_update_co
 
     assert_includes post.author_addresses, address
     post.author_addresses.delete(address)
-    assert_predicate post[:author_count], :nil?
+    assert_nil post[:author_count]
   end
 
   def test_primary_key_option_on_source

railties/test/application/configuration_test.rb

@@ -4028,7 +4028,7 @@ class Post < ActiveRecord::Base
 
       app "development"
 
-      assert_equal nil, ActiveStorage.variant_processor
+      assert_nil ActiveStorage.variant_processor
     end
 
     test "ActiveStorage.variant_processor uses vips by default" do