Do not always pass in user_data to callbacks.
Do not always pass in user_data to callbacks.

This keeps API compatibility with PyGTK and avoids sending
in user_data if it's None.

https://bugzilla.gnome.org/show_bug.cgi?id=653462

thiblahute/pygobject,davidmalcolm/pygobject,choeger/pygobject-cmake,GNOME/pygobject,thiblahute/pygobject,sfeltman/pygobject,davibe/pygobject,pexip/pygobject,choeger/pygobject-cmake,jdahlin/pygobject,jdahlin/pygobject,MathieuDuponchelle/pygobject,davibe/pygobject,sfeltman/pygobject,davidmalcolm/pygobject,jdahlin/pygobject,MathieuDuponchelle/pygobject,davidmalcolm/pygobject,pexip/pygobject,thiblahute/pygobject,pexip/pygobject,davibe/pygobject,GNOME/pygobject,sfeltman/pygobject,davibe/pygobject,choeger/pygobject-cmake,MathieuDuponchelle/pygobject,GNOME/pygobject