Handle Mongo collection fields not being present [DBZ-4882]

Migrated from [DBZ-4882](https://issues.redhat.com/browse/DBZ-4882)

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.

Start by locating the implementation of transforms.outbox.collection.fields.additional.placement and the Mongo collection-field handling it invokes. Trace how an absent field currently becomes an error, then identify the relevant connector tests or test entry point and verify that the chosen configuration writes a null additional field instead.

Written by the indexing model from the issue text.