tools: ynl-gen: fill in missing empty attr lists

The C codegen refers to op attribute lists all over the place,
without checking if they are present, even tho attribute list
is technically an optional property. Add them automatically
at init if missing so that we don't have to make specs longer.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250429154704.2613851-4-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>