[OSX] GetFrameSize - explicitly detect when native side didnt touch result object to signal null.

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

@@ -205,11 +205,7 @@ HRESULT WindowBaseImpl::GetFrameSize(AvnSize *ret) {
         if (ret == nullptr)
             return E_POINTER;
 
-        if(Window == nullptr){
-            ret->Width = 0;
-            ret->Height = 0;
-        }
-        else {
+        if(Window != nullptr){
             auto frame = [Window frame];
             ret->Width = frame.size.width;
             ret->Height = frame.size.height;

src/Avalonia.Native/WindowImplBase.cs

@@ -116,8 +116,12 @@ namespace Avalonia.Native
             {
                 if (_native != null)
                 {
-                    var s = _native.FrameSize;
-                    return s.Width == 0 && s.Height == 0 ? null : new Size(s.Width, s.Height);
+                    unsafe
+                    {
+                        var s = new AvnSize { Width = -1, Height = -1 };
+                        _native.GetFrameSize(&s);
+                        return s.Width < 0  && s.Height < 0 ? null : new Size(s.Width, s.Height);
+                    }
                 }
 
                 return default;

src/Avalonia.Native/avn.idl

@@ -504,7 +504,7 @@ interface IAvnWindowBase : IUnknown
      HRESULT Close();
      HRESULT Activate();
      HRESULT GetClientSize(AvnSize*ret);
-     HRESULT GetFrameSize(AvnSize*ret);
+     HRESULT GetFrameSize(AvnSize*result);
      HRESULT GetScaling(double*ret);
      HRESULT SetMinMaxSize(AvnSize minSize, AvnSize maxSize);
      HRESULT Resize(double width, double height, AvnPlatformResizeReason reason);