You can setup a microflow in your mapping to handle this case.
If you double click the entity, you can change the method from create to microflow, which will give you full flexibility about what values will be written.
To add onto Austin's answer. I have built out the microflow he's mentioning.
See my answer on this (similar) question: https://forum.mendix.com/link/questions/92121
You can set it to send empty values in the Properties tab of the Export mapping. I checked and this is available in 7.8 – it should be available in Modeler 7.11 as well. :)