Don't return empty user session if possible

[Michael Terry]

This is not critical and is easily worked around by greeters. But if a user does not have a stored preferred session, liblightdm will return an empty string when asked for their session.

Maybe it would be nicer to greeters if liblightdm returned the default session instead in that case. I don't *think* a greeter ever would want to differentiate between those cases itself.

[Robert Ancell (robert-ancell)]

Marking won't fix because the current behaviour is the intended and is documented as such. Any change could potentially break a greeter. I think it is possible (though probably unlikely) a greeter might want to visually indicate you have picked a session as opposed getting whatever the default is (or the greeter might want to pick the default).

Acknowledge this is a bit harder than liblightdm doing it all for you. I think if a simpler "common behaviour" API was required this could be made in liblightdm with a SimpleGreeter class (probably not required given there aren't that many greeters).