Properly handle empty CSGCombiners

(cherry picked from commit 001b6075cb148d2300cbbf1e65c6b7cae19b3b25)

modules/csg/csg_shape.cpp

@@ -655,8 +655,7 @@ CSGShape::~CSGShape() {
 //////////////////////////////////
 
 CSGBrush *CSGCombiner::_build_brush() {
-
-	return NULL; //does not build anything
+	return memnew(CSGBrush); //does not build anything
 }
 
 CSGCombiner::CSGCombiner() {