SDL_SetWindowShape(): Remove shape when argument is NULL

(cherry picked from commit 53c799a7a9790bbf31f781cffffacef4f6bd5fa3)

src/video/SDL_video.c

@@ -6162,9 +6162,13 @@ bool SDL_SetWindowShape(SDL_Window *window, SDL_Surface *shape)
         return false;
     }
 
-    surface = SDL_ConvertSurface(shape, SDL_PIXELFORMAT_ARGB32);
-    if (!surface) {
-        return false;
+    if (shape) {
+        surface = SDL_ConvertSurface(shape, SDL_PIXELFORMAT_ARGB32);
+        if (!surface) {
+            return false;
+        }
+    } else {
+        surface = NULL;
     }
 
     if (!SDL_SetSurfaceProperty(props, SDL_PROP_WINDOW_SHAPE_POINTER, surface)) {