cpuset: remove redundant special case for null input in node mask update

The nodelist_parse function already handles empty nodemask input
appropriately, making it unnecessary to handle this case separately
during the node mask update process.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>