Make sure errors.messages works in the same way as Rails 6.1

When there are no errors for a given attribute we were returning empty
arrays. We should continue to do that.

Author: rafaelfranca

activemodel/lib/active_model/errors.rb

@@ -228,18 +228,22 @@ def to_hash(full_messages = false)
 
     undef :to_h
 
+    EMPTY_ARRAY = [].freeze # :nodoc:
+
     # Returns a Hash of attributes with an array of their error messages.
     def messages
       hash = to_hash
+      hash.default = EMPTY_ARRAY
       hash.freeze
+      hash
     end
 
     # Returns a Hash of attributes with an array of their error details.
     def details
       hash = group_by_attribute.transform_values do |errors|
         errors.map(&:details)
       end
-      hash.default = []
+      hash.default = EMPTY_ARRAY
       hash.freeze
       hash
     end

activemodel/test/cases/errors_test.rb

@@ -455,6 +455,14 @@ def test_no_key
     assert_nil person.errors.as_json.default_proc
   end
 
+  test "messages returns empty frozen array when when accessed with non-existent attribute" do
+    errors = ActiveModel::Errors.new(Person.new)
+
+    assert_equal [], errors.messages[:foo]
+    assert_raises(FrozenError) { errors.messages[:foo] << "foo" }
+    assert_raises(FrozenError) { errors.messages[:foo].clear }
+  end
+
   test "full_messages doesn't require the base object to respond to `:errors" do
     model = Class.new do
       def initialize
@@ -621,6 +629,8 @@ def call
     errors = ActiveModel::Errors.new(Person.new)
 
     assert_equal [], errors.details[:foo]
+    assert_raises(FrozenError) { errors.details[:foo] << "foo" }
+    assert_raises(FrozenError) { errors.details[:foo].clear }
   end
 
   test "copy errors" do