Add a missing test case for the persistence behavior of `serialize`

`serialize` makes the contract that if it is given a class name, it will
never return something other than an instance of that class. This means
that it must cast `nil` to the empty form of that object. As such, we
should then persist empty forms of that object as `nil`. While this is
techincally under the contract of

```
model.attribute = value
assert_equal model.attribute, model.tap(&:save).reload.attribute
```

which we can't actually test universally without property based testing,
it has come up more than once and is worth calling out specifically
since we aren't looking to change it.

1 files changed, 13 insertions, 0 deletions

diff --git a/activerecord/test/cases/serialized_attribute_test.rb b/activerecord/test/cases/serialized_attribute_test.rb
index 7c92453ee3..a31c3c828c 100644
--- a/activerecord/test/cases/serialized_attribute_test.rb
+++ b/activerecord/test/cases/serialized_attribute_test.rb
@@ -274,4 +274,17 @@ class SerializedAttributeTest < ActiveRecord::TestCase
 
     assert_equal({}, topic.content)
   end
+
+  def test_values_cast_from_nil_are_persisted_as_nil
+    # This is required to fulfil the following contract, which must be universally
+    # true in Active Record:
+    #
+    # model.attribute = value
+    # assert_equal model.attribute, model.tap(&:save).reload.attribute
+    Topic.serialize(:content, Hash)
+    topic = Topic.create!(content: {})
+    topic2 = Topic.create!(content: nil)
+
+    assert_equal [topic, topic2], Topic.where(content: nil)
+  end
 end