tools: ynl-gen: don't consider requests with fixed hdr empty

C codegen skips generating the structs if request/reply has no attrs.
In such cases the request op takes no argument and return int
(rather than response struct). In case of classic netlink a lot of
information gets passed using the fixed struct, however, so adjust
the logic to consider a request empty only if it has no attrs _and_
no fixed struct.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250410014658.782120-9-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>