Fixes #9975.

This PR fixes an error that can occur when the streaming pull manager is being shut down when the underlying RPC gets shut down.

When that happens, the StreamingPullManager's _on_response() method is invoked with None (as opposed to a StreamingPullResponse instance). This commit handles this case gracefully.

How to test

(this is how I managed to reproduce the error on my machine consistently)

• Use PubSub v1.1.0 (the bug does reportedly not occur in an earlier version, e.g. v1.0.2)
• Create a topic and a subscription to that topic.
• (optional) Set logging level to DEBUG.
• Start pulling messages from the subscription using the subscriber's streaming pull.
• While pulling messages, generate a keyboard interrupt, and cancel the streaming pull future as a response to the interrupt.
  (alternative: close the channel manually: subscriber.api.transport.channel.close()

Actual result (before the fix):
An error occurs during the shutdown ("'NoneType' object has no attribute 'received_messages'") in one of the background threads.

Expected result (after the fix):
No error occurs, and the streaming pull shuts down cleanly (check the DEBUG log output for details).

A sketch of the test script:

subscriber = pubsub_v1.SubscriberClient()
future = subscriber.subscribe(SUBSCRIPTION_PATH, callback=lambda msg: None)

try:
    future.result()
except KeyboardInterrupt:
    future.cancel()

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)