CollectionMetadata::keyBelongsToMe() should assert that given key is non-empty in debug builds

mongos enforces that documents modified in or inserted into sharded collections must have non-empty values for the shard key (though this is not enforced if mongos is bypassed). CollectionMetadata::keyBelongsToMe() could have a debug-only assert that fails the operation or terminates the process if a document is encountered that is missing the shard key.