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

```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
$
```

Refer to
https://github.com/minitest/minitest/blob/master/History.rdoc#5230--2024-05-15-
minitest/minitest@f0f17b99

Author: yahonda

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