splitmuxsrc: don't respond to duration query with CLOCK_TIME_NONE
splitmuxsrc: don't respond to duration query with CLOCK_TIME_NONE

total_duration is initialised to CLOCK_TIME_NONE, not 0, so check
for that as well in order not to return an invalid duration to
a duration query. Doesn't fix anything particular observed in
practice, just seemed inconsistent.

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