netdev-genl: avoid empty messages in napi get

Empty netlink responses from do() are not correct (as opposed to
dump() where not dumping anything is perfectly fine).
We should return an error if the target object does not exist,
in this case if the netdev is down we "hide" the NAPI instances.

Fixes: 27f91aaf ("netdev-genl: Add netlink framework functions for napi")
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241219032833.1165433-1-kuba@kernel.org


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