cephadm: add "assert foo is not None" for mypy check

cephadm: add "assert foo is not None" for mypy check

it's legit to pass file objects to fcntl(), but `Popen.stdout` and
`Popen.stderr` properies are not necessarily file objects -- they could be None.
this cannot be deduced at compile-time. even we can ensure this,
as we do pass `subprocess.PIPE` to the constructor. so mypy just
complains at seeing this:

```
cephadm:429: error: Argument 1 to "fcntl" has incompatible type "Optional[IO[Any]]"; expected "Union[int, HasFileno]"
cephadm:430: error: Argument 1 to "fcntl" has incompatible type "Optional[IO[Any]]"; expected "Union[int, HasFileno]"
cephadm:431: error: Argument 1 to "fcntl" has incompatible type "Optional[IO[Any]]"; expected "Union[int, HasFileno]"
cephadm:432: error: Argument 1 to "fcntl" has incompatible type "Optional[IO[Any]]"; expected "Union[int, HasFileno]"
cephadm:455: error: Item "None" of "Optional[IO[Any]]" has no attribute "fileno"
cephadm:465: error: Item "None" of "Optional[IO[Any]]" has no attribute "fileno"
cephadm:475: error: Item "None" of "Optional[IO[Any]]" has no attribute "fileno"
```

to silence this warning, insert `assert process.stdout is not None`
before accessing `process.stdout` to appease the strict optional
checking of mypy.

this is a follow-up fix of ee3fe37158422902162257c123ea234da999c961

Fixes: https://tracker.ceph.com/issues/44557
Signed-off-by: Kefu Chai <kchai@redhat.com>