avformat/dashenc: fix writing the AV1 codec string in mp4 mode

From https://aomediacodec.github.io/av1-isobmff/#codecsparam

, the parameters
sample entry 4CC, profile, level, tier, and bitDepth are all mandatory fields.
All the other fields are optional, mutually inclusive (all or none).

Fixes ticket #8049

Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 1cf2f040)