util/box: transform empty boxes' origins

Not a huge fan of this, but changing this behavior would be a breaking change
to a stable API.

Fixes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3650

util/box.c

@@ -71,13 +71,11 @@ bool wlr_box_contains_point(const struct wlr_box *box, double x, double y) {
 
 void wlr_box_transform(struct wlr_box *dest, const struct wlr_box *box,
 		enum wl_output_transform transform, int width, int height) {
-	if (wlr_box_empty(box)) {
-		*dest = (struct wlr_box){0};
-		return;
+	struct wlr_box src = {0};
+	if (box != NULL) {
+		src = *box;
 	}
 
-	struct wlr_box src = *box;
-
 	if (transform % 2 == 0) {
 		dest->width = src.width;
 		dest->height = src.height;
@@ -128,13 +126,11 @@ bool wlr_fbox_empty(const struct wlr_fbox *box) {
 
 void wlr_fbox_transform(struct wlr_fbox *dest, const struct wlr_fbox *box,
 		enum wl_output_transform transform, double width, double height) {
-	if (wlr_fbox_empty(box)) {
-		*dest = (struct wlr_fbox){0};
-		return;
+	struct wlr_fbox src = {0};
+	if (box != NULL) {
+		src = *box;
 	}
 
-	struct wlr_fbox src = *box;
-
 	if (transform % 2 == 0) {
 		dest->width = src.width;
 		dest->height = src.height;