PD: allow to clear the shape when nullifying the Support property

src/Mod/PartDesign/App/ShapeBinder.cpp

@@ -125,6 +125,9 @@ App::DocumentObjectExecReturn* ShapeBinder::execute(void) {
             this->Placement.setValue(shape.getTransform());
             this->Shape.setValue(shape);
         }
+        else {
+            this->Shape.setValue(shape);
+        }
     }
 
     return Part::Feature::execute();