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

addresses #125

@wing328 I hope that works for you. If you want any changes, please let me know.

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Summary by cubic

Blank strings now deserialize to JsonNullable.of(null) (present null) instead of JsonNullable.undefined() for non-String types. Fixes #125 and aligns behavior with default deserialization.

• Bug Fixes
  • Removed the empty-string check from JsonNullableJackson2Deserializer and JsonNullableJackson3Deserializer to let blank strings coerce to null.
  • Updated tests to expect JsonNullable.of(null) for empty or whitespace strings; missing fields still yield JsonNullable.undefined().