doc/netlink: Allow empty enum-name in ynl specs

Update the ynl schemas to allow the specification of empty enum names
for all enum code generation.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240306231046.97158-6-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>