UnwrapFromMongoDbEnvelope fails when encountering Tombstone messages

Context

When a document is deleted on a MongoDB Collection 2 messages will appear:

• First being a delete operation as in the oplog

  SourceRecord{sourcePartition={rs=rs0, server_id=mongo}, sourceOffset={sec=1542546121, ord=3, h=8008529886791156268}} ConnectRecord{topic='mongo.transform.source', kafkaPartition=null, key=Struct{id=1}, value=Struct{source=Struct{version=0.9.0-SNAPSHOT,connector=mongodb,name=mongo,rs=rs0,ns=transform.source,sec=1542546121,ord=3,h=8008529886791156268},op=d,ts_ms=1542546121897}, timestamp=null, headers=ConnectHeaders(headers=)}
  

• Second a Tombstone message which value is null

  SourceRecord{sourcePartition={rs=rs0, server_id=mongo}, sourceOffset={sec=1542546121, ord=3, h=8008529886791156268}} ConnectRecord{topic='mongo.transform.source', kafkaPartition=null, key=Struct{id=1}, value=null, timestamp=null, headers=ConnectHeaders(headers=)}
  

How to reproduce

In a connector with UnwrapFromMongoDbEnvelope enabled, delete a document within the collection.

The result is as follows:

java.lang.NullPointerException
	at io.debezium.connector.mongodb.transforms.UnwrapFromMongoDbEnvelope.apply(UnwrapFromMongoDbEnvelope.java:125)
	at io.debezium.connector.mongodb.transforms.UnwrapFromMongoDbEnvelopeTestIT.shouldTransformEvents(UnwrapFromMongoDbEnvelopeTestIT.java:148)

Reason

It happens because the Tombstone message has no value nor valueSchema, this part of the transformer UnwrapFromMongoDbEnvelopeTestIT.java:148 depends on the valueSchema to exist which isn't true for the tombstone