net: netlink: catch attempts to send empty messages

syzbot can figure out a way to redirect a netlink message to a tap.
Sending empty skbs to devices is not valid and we end up hitting
a skb_assert_len() in __dev_queue_xmit().

Make catching these mistakes easier, assert the skb size directly
in netlink core.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241218024400.824355-1-kuba@kernel.org


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