Delete Documents with mongo connect-sink does not work

Hi everyone,

I am working in a project where I publish data to a kafka topic(in AVRO format), and I want to use a Mongo-Sink Connector in order to UPSERT documents and DELETE a document when I publish a null value to the topic.
The topic will have as a key the values that I want to use in order to find and update a document, but also the values that I can use to find and delete a document, in case I am publishing null value to the topic.
The topic has the following format:

**key**: { "value_1":"xyz", "value_2":100, "value_3":1000 } 
**value**: {
  "value_1": "xyz",
  "value_2": 100,
  "value_3": 1000,
  "value_4": 3000,
  "value_5": true,

When I want to delete values, then I send the following in my kafka topic:

**key**: { "value_1":"xyz", "value_2":100, "value_3":1000 }
**value**:  null

UPSERT operation works completely fine, but DELETE it does not work at all.
I am trying to understand what I am doing wrong.

You can see next the configuration I have for my mongo-sink-connector:

    "name": "mongodb-sink-test",
    "config": {

Any ideas of what I am doing wrong ?

Appreciate your help !