Revert "Revert "Add missing null check to String::ToCharArray""

Fix flaky test.

Bug: 25641543

This reverts commit b52a39c76e0cb0638c92cdcf6b5c5aa551420e1a.

Change-Id: I6b0d3087dba3680252e65af554530e2769cde55c
3 files changed