The MongoDb connector allows users to specify a list of fields that are to be renamed in the output message using the field.renames configuration option. When this configuration is used and the field to be renamed is not in the original output message, the connector will add the replacement field to the output message regardless with a null value.

Per the conversation https://gitter.im/debezium/dev?at=5e5fbada53fa513e286b8996, we agree that this should not occur and instead the field replacement should only apply if the field to be replaced exists.