Deserialising an empty character as json null instead of json undefined

Recently we changed a type in one object from JsonNullable<String> sth = JsonNullable.undefined() to JsonNullable<Character> sth = JsonNullable.undefined(). Soon after we noticed a change in the deserialisation behaviour - explicit updates of that property to an empty string ("") no longer worked, as they were treated as not present (JsonNullable.undefined instead of JsonNullable[null]).

It seems that the issue is in the JsonNullableDeserializer, namely in the constructor:

this.isStringDeserializer = ((ReferenceType) fullType).getReferencedType().isTypeOrSubTypeOf(String.class);

Does it make sense to change the check and include Character as well as String? Or would you suggest another alternative? If it is the former, we can also open a PR, as the change doesn't seem so large (and there is already https://github.com/OpenAPITools/jackson-databind-nullable/pull/4 for inspiration).

Start with the JsonNullableDeserializer constructor and inspect how isStringDeserializer is set for String and Character reference types. Compare the proposed change with pull request #4, then verify that deserialising an explicit empty string for JsonNullable produces a present null value rather than JsonNullable.undefined().