flvdemux: Don't write an empty string as a tag
flvdemux: Don't write an empty string as a tag

To stop warnings like:

GStreamer-WARNING **: 19:47:48.186: Trying to set empty string on
taglist field 'encoder'. Please file a bug.

pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good