tracker-extract-gstreamer: Simplify tagcache behaviour
tracker-extract-gstreamer: Simplify tagcache behaviour

Avoid having a NULL tagcache, which allows us to use
gst_tag_list_insert() to merge new tags.

hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker