tracker-extract-gstreamer.c: Set empty tags list if no tags were found

If entry doesn't has tags then tracker_toc_add_entry receives NULL instead of the tag list. But this function immediately tries to make a reference to the tag list and fails on NULL pointer. And every function in tracker-cue-sheet.c assumes that the list is non-NULL.

Now it creates empty tag list if an entry doesn't contain tags. It shouldn't have any impact since in normal case tag list is copied.