Fixed an NPE on null map and list claims

[DanilaVaratyntsev]

Changes

Added a null-check before map and list validation in withClaim methods to support the same behavior, like other withClaim methods (remove a claim on a not-null key but null value)

References

#416

Testing

• Added unit tests for null list and map claims
• Built library locally, imported it to the testing project and run the following code

String jwt = JWT.create()
    .withClaim("mapClaim", (Map<String, ?>) null)
    .withClaim("listClaim", (List<String>) null)
    .sign(Algorithm.HMAC256("secret"));

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[lbalmaceda]

This looks good. Thanks for taking care of that 🥇