Don't return FrameSize until window is shown.

Since #8629, the content size isn't set until the window is shown, so we can't know the frame size.

native/Avalonia.Native/src/OSX/WindowBaseImpl.mm

@@ -224,7 +224,7 @@ HRESULT WindowBaseImpl::GetFrameSize(AvnSize *ret) {
         if (ret == nullptr)
             return E_POINTER;
 
-        if(Window != nullptr){
+        if(Window != nullptr && _shown){
             auto frame = [Window frame];
             ret->Width = frame.size.width;
             ret->Height = frame.size.height;