support strings empty return null

[thinking-github]

support strings empty return null

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[eamonnmcmanus]

Can you say something more about the problem you are trying to solve here? Right now we just have a bunch of code with a 5-word description. What is it that people will be able to do with this change that they can't do today?

Also, a reminder that Gson is in maintenance mode and we are generally reluctant to accept new features.