python-common/service_spec: fix some mypy complaints

python-common/service_spec: fix some mypy complaints

The python/mypy combination on the jenkins nodes the CI
is running on don't seem to care, but locally I get

mypy: commands[0]> mypy --config-file=../mypy.ini -p ceph
ceph/deployment/service_spec.py: note: In member "validate" of class "NvmeofServiceSpec":
ceph/deployment/service_spec.py:1497: error: Unsupported operand types for > ("float" and "None")  [operator]
ceph/deployment/service_spec.py:1497: note: Left operand is of type "Optional[float]"
ceph/deployment/service_spec.py:1500: error: Unsupported operand types for > ("int" and "None")  [operator]
ceph/deployment/service_spec.py:1500: note: Left operand is of type "Optional[int]"
ceph/deployment/service_spec.py:1503: error: Unsupported operand types for > ("int" and "None")  [operator]
ceph/deployment/service_spec.py:1503: note: Left operand is of type "Optional[int]"
ceph/deployment/service_spec.py:1506: error: Unsupported operand types for > ("int" and "None")  [operator]
ceph/deployment/service_spec.py:1506: note: Left operand is of type "Optional[int]"
ceph/deployment/service_spec.py:1509: error: Unsupported operand types for > ("int" and "None")  [operator]
ceph/deployment/service_spec.py:1509: note: Left operand is of type "Optional[int]"
ceph/deployment/service_spec.py:1512: error: Unsupported operand types for > ("int" and "None")  [operator]
ceph/deployment/service_spec.py:1512: note: Left operand is of type "Optional[int]"
ceph/deployment/service_spec.py:1515: error: Unsupported operand types for > ("float" and "None")  [operator]
ceph/deployment/service_spec.py:1515: note: Left operand is of type "Optional[float]"
ceph/deployment/service_spec.py:1518: error: Unsupported operand types for > ("int" and "None")  [operator]
ceph/deployment/service_spec.py:1518: note: Left operand is of type "Optional[int]"
ceph/deployment/service_spec.py:1521: error: Unsupported operand types for > ("int" and "None")  [operator]
ceph/deployment/service_spec.py:1521: note: Left operand is of type "Optional[int]"
ceph/deployment/service_spec.py:1524: error: Unsupported operand types for > ("int" and "None")  [operator]
ceph/deployment/service_spec.py:1524: note: Left operand is of type "Optional[int]"
ceph/deployment/service_spec.py:1527: error: Unsupported operand types for > ("int" and "None")  [operator]
ceph/deployment/service_spec.py:1527: note: Left operand is of type "Optional[int]"
ceph/deployment/service_spec.py:1530: error: Unsupported operand types for > ("float" and "None")  [operator]
ceph/deployment/service_spec.py:1530: note: Left operand is of type "Optional[float]"
Found 12 errors in 1 file (checked 27 source files)

The errors make sense to me, so I think we should fix them

Signed-off-by: Adam King <adking@redhat.com>