windowing: workaround for old config files

windowing: workaround for old config files

Reported by deevad via IRC:
File "/usr/local/share/mypaint/gui/windowing.py", line 46, in window_factory
    module = __import__(role.lower(), globals(), locals(), [])
ImportError: No module named null

Not sure why exactly we get 'null' here, but it's the obvious fix to
replace the old catch-all exception handler from 780ba0.