avformat/matroskaenc: Only write Tracks if there is a track

avformat/matroskaenc: Only write Tracks if there is a track

The Matroska muxer does not write every stream as a Matroska track;
some streams are written as AttachedFile. But should no stream be
written as a Matroska track, the Matroska muxer would nevertheless
write a Tracks element without a TrackEntry. This is against the spec.
This commit changes this and only writes a Tracks if there is a Matroska
track.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>