Guard against NullPointerException when getting Credentials from Json

[bennycao]

Changes

This change adds code to guard against possible NullPointerException thrown as a result of f being null in the JsonRequiredTypeAdaptor

Solves https://github.com/auth0/Auth0.Android/issues/676

Testing

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