master.cfg: fix exception due to empty branch-property
master.cfg: fix exception due to empty branch-property

Probably this happend, because buildbot-api changed over time.
The hint for solving that issue was found here:
https://github.com/buildbot/buildbot/issues/4288\#issuecomment-683210228

Apparently, the property branch only gets set, if it is not
the default value. That was fixed by adding an exception
for that case.

freifunk-berlin/buildbot