tools: ynl: add ynl_dump_empty() helper

Checking if dump is empty requires a couple of casts.
Add a convenient wrapper.

Add an example use in the netdev sample, loopback is always
present so an empty dump is an error.

Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240329181651.319326-1-kuba@kernel.org


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