Examples from "Form Optional / Required" section of the form reference: if the field simple type does not admit the "empty string" use ctrl.emptyisnull=false and form.empty=false, try to change to true to see the empty string as an accepted value

Field String Optional(Optional)Standard optional field with maybenull=true and extra:ctrl.emptyisnull=false, value space admits the empty string so we must use form.empty=false to make form field invalid when empty