zuul.gerrit.Gerrit.isMerged should not return None

* zuul/trigger/gerrit.py(Gerrit.isMerged): Make sure to return False
if we fell through all the conditionals, rather than defaulting to
None. This should have no current impact since all comparisons to
the returned value elsewhere are boolean and not "is None" or
similar.

Change-Id: I05360833f1e5b35fbfb1894dadcb46e82ff78a11
Merge "zuul.gerrit.Gerrit.isMerged should not return None"