Don't treat `nil` as changed in serialized types

We were ignoring the `default_value?` escape clause in the serialized
type, which caused the default value to always be treated as changed.

Fixes #18169

1 files changed, 8 insertions, 0 deletions

diff --git a/activerecord/test/cases/serialized_attribute_test.rb b/activerecord/test/cases/serialized_attribute_test.rb
index 56a0e92e1d..c8441201ca 100644
--- a/activerecord/test/cases/serialized_attribute_test.rb
+++ b/activerecord/test/cases/serialized_attribute_test.rb
@@ -256,4 +256,12 @@ class SerializedAttributeTest < ActiveRecord::TestCase
     assert_equal("second", t.content)
     assert_equal("second", t.reload.content)
   end
+
+  def test_nil_is_not_changed_when_serialized_with_a_class
+    Topic.serialize(:content, Array)
+
+    topic = Topic.new(content: nil)
+
+    assert_not topic.content_changed?
+  end
 end