fix: flatten enum inside Option

[meskill]

fixes #317

[GREsau]

I'm afraid the schemas generated by this change are not correct. For the flatten_option_enum.json schema from the tests:

• one of the subschemas has "type": null which is invalid in that context - presumably you intended it to be "type": "null"?
• even with that fixed, the empty object {} does not successfully validate against the schema when it should (because it can successfuly deserialize to FlattenOptionEnum { enum_: None })