Dangling awaitable when shutting down asynchronous Client

When shutting down an asynchronous client, `self.close()` might return a `NoOpAwaitable `(if for example the client has been manually closed). This however results in a dangling awaitable, giving warnings.
It's also strange that while a asynchronous client is 'closing', a `None` type is returned, and not a `NoOpAwaitable `.
https://github.com/dask/distributed/blob/40456b46598a1b096a580af6ad60c7bb6852a3b1/distributed/client.py#L1467-L1471