Clarify the behaviour when the input string is null.

Based on a patch written by Erik van der Poel.

Change-Id: Ia14e3ebae79a008372bdc28f763c0abcc6f4b3a3
Reviewed-on: https://code-review.googlesource.com/3900
Reviewed-by: Paul Wankadia <junyer@google.com>
2 files changed