lavf/matroskaenc: Check for valid metadata before creating tags

Tags must have at least one SimpleTag element to be spec conformant.
Updated lavf-mkv and seek-lavf-mkv FATE references as the tests were affected by
this.

Fixes ticket #2785

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 088ed5314694031e07e02e9d364c9d486a60e704)
matroskaenc: simplify mkv_check_tag()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 066111bf19518a9f4d836991b34dbfc5ab72a41a)