roslaunch: fix handling of if/unless attributes on args. Fix #437.

In cases where none of the conditionals were satisfied, _parse_arg(..)
did not return a value, which caused _parse_subcontext(..) to fail,
as it tried to unpack the None.
Merge pull request #439 from gavanderhoorn/issue437_if_unless_none

roslaunch: fix if/unless attribute handling