netlink: Add nla_put_empty_nest helper

Creating empty nests is helpful when the exact attributes to be exposed
in the future are not known. Encapsulate the logic in a helper.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250214211255.14194-2-jdamato@fastly.com


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