JSON Schema gets a second null option when a #[JsonSchema] attribute allows null without a type

For optional data, getJsonSchemaForMethod() adds a {"type": "null"} option in a oneOf. If the declared #[JsonSchema] already accepts NULL, this results in a double NULL, which is incorrect.

The fix asks the justinrainbow/json-schema validator, which core ships with, whether NULL validates against the declared schema, and only adds the null option when it doesn't.