check for empty breadcrumb the right way

[nick-mccoy]

Description of change

When validating the state, check for an empty breadcrumb by checking for an empty list instead of an empty tuple.

Manual QA steps

Had a client that was stuck in an infinite loop since they de-selected the stream that was bookmarked as currently_syncing. Was able to verify that this fix takes them out of the looping behavior.

Risks

None since this function was not working correctly before

Rollback steps

Revert this change

[dmosorast]

This does look like the correct behavior, since the state and catalog will be loaded from JSON, which doesn't have the concept of a tuple. Unit tests should be updated to reflect this.