Avoid harmless warning trying to split None autostartcondition

When self.get_austostart_condition() returns None and
autostart_type is not None, the following code will
try to apply split on None.
The exception handling catches this and returns the correct
value for the function in this case so it's harmless,
this just fixes so that there's no traceback in the application
output....

https://bugzilla.gnome.org/show_bug.cgi?id=702994