media_metadata: ensure we always call the user's callback
media_metadata: ensure we always call the user's callback

Even when there is no source to resolve keys.

Signed-off-by: Lionel Landwerlin <9463032a5b99ffad10f36631678a833369a311b5@linux.intel.com>

GNOME/grilo,kyoushuu/grilo,jasuarez/grilo,grilofw/grilo,GNOME/grilo,MathieuDuponchelle/grilo,MathieuDuponchelle/grilo,kyoushuu/grilo,jasuarez/grilo,kyoushuu/grilo,jasuarez/grilo,grilofw/grilo,MathieuDuponchelle/grilo,kyoushuu/grilo,grilofw/grilo