Prevent invalid warnings about missing identifiers

Prevent invalid warnings about missing identifiers

The Danger script inspects differences between the current version of a
given merge request's target branch and the merge request branch.  If
the latter falls behind the former, the Danger script will wrongly warn
about missing GitLab/RT identifiers because it incorrectly treats the
"+++" diff marker as an indication of the merge request adding new lines
to a file.  Tweak the relevant conditional expression to prevent such
invalid warnings from being raised.

(cherry picked from commit e062812c38787dbd37c95c6dda1d32c9dce6a3c8)