pybind/mgr/status: drop asserts that fight the defaultdict defaults

pybind/mgr/status: drop asserts that fight the defaultdict defaults

The 'assert metadata' checks in the status module were actually fighting
against our own defaults. Since an empty defaultdict is falsy, these
asserts would blow up the whole command if a single daemon was down
after a mgr restart.

This drops those four grumpy asserts. Now, instead of a traceback,
`ceph osd status` and `ceph fs status` will just show a blank hostname
 or "unknown" version as intended.

The trigger is common in practice: any mgr restart leaves daemons
that are currently down without metadata in daemon_state, since
they never reconnect via MMgrOpen to repopulate it. After such a
restart, `ceph osd status` and `ceph fs status` blow up:
```
  Error EINVAL: Traceback (most recent call last):
    ...
    File ".../status/module.py", line 340, in handle_osd_status
      assert metadata
  AssertionError
```

The bug was introduced in 5ac2901f54ff

Fixes: https://tracker.ceph.com/issues/76416
Reported-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Signed-off-by: Kefu Chai <tchaikov@gmail.com>