Make sure to persist a newly-nil serialized value

The subtype will (quite reasonably) ignore the possibility that it has
`changed_in_place?` by becoming nil.

Fixes #19467

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Correctly persist a serialized attribute that has been returned to
+    its default value by an in-place modification.
+
+    Fixes #19467.
+
+    *Matthew Draper*
+
 *   Fix generating the schema file when using PostgreSQL `BigInt[]` data type.
     Previously the `limit: 8` was not coming through, and this caused it to
     become `Int[]` data type after rebuilding from the schema.

activerecord/lib/active_record/type/serialized.rb

@@ -28,7 +28,9 @@ module ActiveRecord
 
       def changed_in_place?(raw_old_value, value)
         return false if value.nil?
-        subtype.changed_in_place?(raw_old_value, serialize(value))
+        raw_new_value = serialize(value)
+        raw_old_value.nil? != raw_new_value.nil? ||
+          subtype.changed_in_place?(raw_old_value, raw_new_value)
       end
 
       def accessor

activerecord/test/cases/serialized_attribute_test.rb

@@ -264,4 +264,14 @@ class SerializedAttributeTest < ActiveRecord::TestCase
       Topic.serialize(:content, Regexp)
     end
   end
+
+  def test_newly_emptied_serialized_hash_is_changed
+    Topic.serialize(:content, Hash)
+    topic = Topic.create(content: { "things" => "stuff" })
+    topic.content.delete("things")
+    topic.save!
+    topic.reload
+
+    assert_equal({}, topic.content)
+  end
 end