Backport of "rtputils: Count metas with an empty tag list for copying/keeping" into 1.18
requested to merge arun/gst-plugins-good:backport-mr-779-into-1.18-rtputils-keep-and-copy-empty-tag-metas into 1.18
Backport of !779 (merged) "rtputils: Count metas with an empty tag list for copying/keeping"