UnwrapFromMongoDbEnvelope doesn't support operation header on tombstone messages

Context

When you try to set the header a tombstone message a NPE happens due to not being able to fetch the field "op" from the Record value

Reproduce

Configure a connector with the Unwrap enabled, set:

"transform.unwrap.operation.header": "true",
"transform.unwrap.drop.tombstones": "false",

Delete one document in the collection, you should expect a NPE