@note the returned string will be av_malloc()ed and must be av_free()ed by the caller
@note if AV_OPT_ALLOW_NULL is set in search_flags in av_opt_get, and the option has
AV_OPT_TYPE_STRING or AV_OPT_TYPE_BINARY and is set to NULL, *out_val will be set
to NULL instead of an allocated empty string.
@note the returned string will be av_malloc()ed and must be av_free()ed by the caller
@note if AV_OPT_ALLOW_NULL is set in search_flags in av_opt_get, and the option has AV_OPT_TYPE_STRING or AV_OPT_TYPE_BINARY and is set to NULL, *out_val will be set to NULL instead of an allocated empty string.