Element templates custom field: camunda:field with no value



when I have an element template that maps a custom field to a cammunda:field value, I cannot load the BPMN process into the workflow engine when no value was set on the custom field in the task.

It seems that the workflow engine assumes that a value must be set in case of a cammunda:field. Also the Modeler shows that it requires a value.

Why is it not possible to have no or emtpy value for a cammunda:field injection?