Do not send an email on reviewes when there is no message.

No longer send an email when reviewing a change via ssh, and
the change message is blank (when no change message is actually
added to the review).  This occured when an approval was
repeated (no change), and no message was explicitly added.

Issue: 1069
Change-Id: Ibf6cd970fdb81ffc1353a2675a2f21a23c46a2cd