streaming_pull_manager.py throws TypeError: object of type 'NoneType' has no len()

[githubwua]

Error exception:

File "/usr/local/lib/python3.10/site-packages/google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py", line 944, in _shutdown

 total = len(dropped_messages) + len(

 TypeError: object of type 'NoneType' has no len()

Thrown by this code:

https://github.com/googleapis/python-pubsub/blame/7cf2d11ce20bf9080c4434b920bdf419faac5d88/google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py#L944

We should add a check to ensure dropped_messages is not None.

[mukund-ananthu]

@githubwua The fix you provide will not help address this issue(whose existence in the latest version of the library is yet to be verified). This is because dropped_messages would be a list(empty or otherwise), and hence a len() on it should not result in an error.

[githubwua]

The proper fix: #1244