avformat/matroskaenc: Don't write empty language

According to the new EBML specifications, a string element of length
zero would be read as the default value by a compliant parser.

Reviewed-by: Ridley Combs <rcombs@rcombs.me>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskaenc.c

@@ -1147,7 +1147,7 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
         put_ebml_string(pb, MATROSKA_ID_TRACKNAME, tag->value);
     tag = av_dict_get(st->metadata, "language", NULL, 0);
     put_ebml_string(pb, MATROSKA_ID_TRACKLANGUAGE,
-                    tag && tag->value ? tag->value : "und");
+                    tag && tag->value[0] ? tag->value : "und");
 
     // The default value for TRACKFLAGDEFAULT is 1, so add element
     // if we need to clear it.