It can be that config.shards has entries without a topologyTime field when the cluster originates from before 5.0.

The ShardRegistry assumes that the topologyTime is Timestamp(0, 1) when none of the entries have the field.

This makes reasoning slightly harder, and would be nice if we could assume there's always a topologyTime in every document in config.shards. And would also be helpful to check consistency of config.shards.