ANDROID: kernel: cgroup: cpuset: Clear cpus_requested for empty buf

update_cpumask had a special case for empty buf which
did not update cpus_requested. This change reduces
differences (only to parsing) in empty/non-empty codepaths
to make them consistent.

Bug: 120444281
Fixes: 4803def4 ("ANDROID: cpuset: Make cpusets restore on hotplug")
Test: check that writes to /dev/cpuset/background/tasks
Test: work as expected, e.g.:
Test: echo $$ > /dev/cpuset/background/tasks
Test: echo > /dev/cpuset/background/tasks
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: I49d320ea046636ec38bd23f053317abc59f64f8e