Blank strings should not be deserialized as JsonNullable.undefined() #125

Context: When a request object has a field like Integer, but an empty/blank string is given, then Jackson deserializes this value as null.
When deserializing the empty/blank string into something like Optional<Integer>, then this will result in a value of Optional.empty(), not null.

The behavior of JsonNullable is inconsistent here: When using JsonNullable<Integer>, an empty/blank string will be deserialized as JsonNullable.undefined() (=not provided in the request) instead of JsonNullable.of(null) (=provided, but a null value).

I think, the best behavior here should be, that empty/blank strings should be deserialized as JsonNullable.of(null), not JsonNullable.undefined()