The primary can be undefined during topology discovery, e.g. during an election. This will cause the RunQueryStats hook to fail here because we directly use the primary here.

https://github.com/10gen/mongo/blob/d93a8e99d098a24e04a4bf89a2170e80aa49b364/jstests/hooks/query_integration/run_query_stats.js#L96

We should check if the primary exists, and if it doesn't we should fall back to using the first node, because this is a similar pattern to what we do in shard_fixture.js: https://github.com/10gen/mongo/blob/d93a8e99d098a24e04a4bf89a2170e80aa49b364/jstests/concurrency/fsm_libs/shard_fixture.js#L63-L68