Handle Mongo collection fields not being present

When the connector is configured to add additional fields as headers using the 

transforms.outbox.collection.fields.additional.placement it would be nice if there was an additional configuration option for configuring how to handle missing fields.

For collections Mongo does not save an entry for a field if it is null and so if the field is included in the transforms.outbox.collection.fields.additional.placement an error is thrown.

It would be nice if you could define this behaviour and just have it write null values in the the additional field instead.