obs-outputs: Skip trak box if track has no data

plugins/obs-outputs/mp4-mux.c

@@ -1718,6 +1718,10 @@ static size_t mp4_write_trak(struct mp4_mux *mux, struct mp4_track *track,
 	struct serializer *s = mux->serializer;
 	int64_t start = serializer_get_pos(s);
 
+	/* If track has no data, omit it from full moov. */
+	if (!fragmented && !track->chunks.num)
+		return 0;
+
 	write_box(s, 0, "trak");
 
 	// tkhd