avfilter/f_metadata: do not return the frame early if there is no metadata

The early return caused isses for the "add" mode (got fixed in
c95dfe5cce) and the "select" mode needs a similar
fix. It is probably better to fully remove the check, since all modes work
correctly with NULL metadata.

Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit 758e2da289)

libavfilter/f_metadata.c

@@ -304,9 +304,6 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *frame)
     AVDictionary **metadata = &frame->metadata;
     AVDictionaryEntry *e;
 
-    if (!*metadata && s->mode != METADATA_ADD)
-        return ff_filter_frame(outlink, frame);
-
     e = av_dict_get(*metadata, !s->key ? "" : s->key, NULL,
                     !s->key ? AV_DICT_IGNORE_SUFFIX: 0);