MongoDB connector throws NPE on "op=n"

The connector only checks for operations of type insert, update, and delete (i, u, d) but if an operation that is not of that type is passed, it will result in a NPE i.e op="n" (no op). The connector should handle such cases gracefully.

https://github.com/confluentinc/debezium/blob/master/debezium-connector-mongodb/src/main/java/io/debezium/connector/mongodb/RecordMakers.java#L182