Instead just Utf-16 encode the null byte when dumping the value to a
string form.
<!--
If you know who should review your pull request, please assign it to
that
person, otherwise the pull request would get assigned randomly.
If your pull request is for a specific language, please add the
appropriate
lang label.
-->
* Detect invalid UTF-8 characters
* Fix checking bug
* Add unit test
* Add more comments and descriptions
* Add corpus
* Small fix to improve readability
* Fix sanity check
Yijie Ma