asfdec: do not export empty metadata

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/asfdec.c

@@ -256,8 +256,10 @@ static int asf_read_metadata(AVFormatContext *s, const char *title, uint16_t len
     AVIOContext *pb = s->pb;
 
     avio_get_str16le(pb, len, ch, buflen);
+    if (ch[0]) {
         if (av_dict_set(&s->metadata, title, ch, 0) < 0)
             av_log(s, AV_LOG_WARNING, "av_dict_set failed.\n");
+    }
 
     return 0;
 }