tools: ynl-gen: fill in implementations for TypeUnused

Fill in more empty handlers for TypeUnused. When 'unused'
attr gets specified in a nested set we have to cleanly
skip it during code generation.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231213231432.2944749-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>