rolling_update: ensure pgs_by_state has at least 1 entry
rolling_update: ensure pgs_by_state has at least 1 entry

Previous commit c13a3c3 has removed a condition.

This commit brings back this condition which is essential to ensure we
won't hit a false positive result in the `when` condition for the check
PGs task.

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>
(cherry picked from commit 179c4d00d702ff9f7a10a3eaa513c289dd75d038)

fgal/ceph-ansible,fgal/ceph-ansible