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

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()

Start at the JsonNullable Jackson deserialization path in this repository and compare how blank strings are handled for Integer, Optional, and JsonNullable. Confirm the change by adding or updating coverage for blank-string input so it yields JsonNullable.of(null) rather than JsonNullable.undefined().