UBI: fix nameless volumes handling

Currently it's possible to create a volume without a name. E.g:
ubimkvol -n 32 -s 2MiB -t static /dev/ubi0 -N ""

After that vtbl_check() will always fail because it does not permit
empty strings.

Cc: stable@kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>