We need the OpenXmlFilter to have the behavior to allow empty targets. As it is now, the source text is always copied into targets, in case they are null.
Apparently, the method getTargetForOutput() inside the class does exactly what we want to avoid (use the source when the target is empty).
Our solution will consist to make this behaviour conditional and add a new flag to the ConditionalParameters class.
Similar behaviour is mentioned in issue #1055 regarding OmegaT with Okapi filter OpenXML.