Fix faux client for unit tests

Now that we use job_config for two use cases we need to cater
for both

Author: withnale

tests/unit/fauxdbi.py

@@ -355,7 +355,7 @@ def __getattr__(self, name):
 class FauxClient:
     def __init__(self, project_id=None, default_query_job_config=None, *args, **kw):
         if project_id is None:
-            if default_query_job_config is not None:
+            if default_query_job_config is not None and default_query_job_config.default_dataset:
                 project_id = default_query_job_config.default_dataset.project
             else:
                 project_id = "authproj"  # we would still have gotten it from auth.