tools: ynl-gen: support empty attribute lists

Differentiate between empty list and None for member lists.
New families may want to create request responses with no attribute.
If we treat those the same as None we end up rendering
a full parsing policy in user space, instead of an empty one.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20230824003056.1436637-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>