UserReference cannot be converted to DocumentReference in Velocity

Steps to reproduce:

In Velocity, call any API that expects a DocumentReference with a UserReference.

Expected result:

The UserReference is converted to a DocumentReference.

Actual result:

The call fails as the conversion fails. This is because the DocumentReferenceConverter calls the UserReferenceConverter to convert to a DocumentReference but this hasn't been implemented in the UserReferenceConverter.