mgr/dashboard: fix service form count bugs

mgr/dashboard: fix service form count bugs

Fixes https://tracker.ceph.com/issues/66608

- for services which do not have a count set default count to be null, otherwise the previous selected service's count is used which is wrong
- make count null when label is selected for placement

Signed-off-by: Afreen Misbah <afreen23.git@gmail.com>
(cherry picked from commit 27a8b2f2883fcc1fc4128b3e8995a7819a5ae51e)

Conflicts:
	src/pybind/mgr/dashboard/frontend/src/app/ceph/cluster/services/service-form/service-form.component.html
	src/pybind/mgr/dashboard/frontend/src/app/core/navigation/navigation/navigation.component.html