[perf] Don't try to send to empty alert email addresses.
[perf] Don't try to send to empty alert email addresses.

Bug: skia:6801
Change-Id: If0ee491bed38f2362c2845cae99be8aa8c97a7b4
Reviewed-on: https://skia-review.googlesource.com/22540
Reviewed-by: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Commit-Queue: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot