Dropping config.shards is allowed and can cause mongos to crash in aggregation code

Perhaps the aggregation code should handle 0 shards?

Or should sharding prevent dropping config.shards? Sharding has discussed before not wanting to disallow direct writes to the config database, since sometimes people do that, and I think mongorestore might drop some of the config collections.

We could also prevent the fuzzer from dropping config collections to prevent the build failures.