It is very easy to misplace a config option to a different level, for
example create an empty replicaset `sharding` with storage role, instead
of configuring sharding option to `storage`:
```
groups:
  g-001:
    replicasets:
      sharding:
        roles:
        - storage
      r-001:
        instances:
          i-001: {}
```

This patch adds validation that forbids creating an empty group or
replicaset. Note that a group or a replicaset could still be defined
in multiple config sources and may appear empty in one of them, the
check is performed on the merged cluster config.

Closes #9895

NO_DOC=bugfix