tools: ynl: don't return None for dumps

YNL currently reports None for empty dump:

 $ cli.py ...netdev.yaml --dump page-pool-get
 None

This doesn't matter for the CLI but when writing YNL based tests
having to deal with either list or None is annoying. Limit the
None conversion to non-dump ops:

 $ cli.py ...netdev.yaml --dump page-pool-get
 []

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240412141436.828666-3-kuba@kernel.org


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