scripts/kernel-doc: ignore identifier on anonymous enums

When anonymous enums are used, the identifier is empty.

While, IMO, it should be avoided the usage of such enums,
adding support for it is not hard.

So, postpone the check for empty identifiers to happen
only at the dump phase.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/055ad57879f1b9381b90879e00f72fde1c3a5647.1614760910.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>