R=agable@chromium.org,iannucci@chromium.org

Before, this block of footers won't be recognized by git_footers:
  Bug:
  Change-Id: xxx

because of empty value for "Bug:". This CL fixes this behavior.

Bug: 715614
Change-Id: Iabe45bfc027fda15cbe0cd5ce9b883ce3b891220
Reviewed-on: https://chromium-review.googlesource.com/487963
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>