tools: ynl-gen-c: improve support for empty nests

tools: ynl-gen-c: improve support for empty nests

Empty nests are the same size as a flag at the netlink level
(just a 4 byte nlattr without a payload). They are sometimes
useful in case we want to only communicate a presence of
something but may want to add more details later.
This may be the case in the upcoming io_uring ZC patches,
for example.

Improve handling of nested empty structs. We already support
empty structs since a lot of netlink replies are empty, but
for nested ones we need minor tweaks to avoid pointless empty
lines and unused variables.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://patch.msgid.link/20250108200758.2693155-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>