Fix AlertToPubSubLiveness
Fix AlertToPubSubLiveness

Only metrics that have a non-empty location make sense to alert on.

Change-Id: I6f93158fee5d465aed39932d23cd7002bea07165
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/273598
Reviewed-by: Weston Tracey <e7e27cb74e46abd7b999223aa4f27901ab7e58c6@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