Fallback for get_queue_stats() when using memory:// broker.

Summary:
When running tests, RabbitMQ is not used, so get_queue_stats()
does not work.

We did not notice this issue before because pytest skipped
importing modules that define tasks with a queue, so this
function was never actually called while running tests.

Reviewers: Reviewers, ardumont

Reviewed By: Reviewers, ardumont

Subscribers: ardumont, swh-public-ci

Differential Revision: https://forge.softwareheritage.org/D763