Garden-o-matic: Do not display empty "[bug ]" list

This would happen for cases like "BUG=None"

TBR=eseidel

Review URL: https://codereview.chromium.org/185653003

git-svn-id: svn://svn.chromium.org/blink/trunk@168286 bbb929c8-8fbe-4397-9dbb-9b2b20218538