[OSX/Avalonia.Native] fix NRE in double dispose of ScreenImpl

src/Avalonia.Native/ScreenImpl.cs

@@ -41,7 +41,7 @@ namespace Avalonia.Native
 
         public void Dispose ()
         {
-            _native.Dispose();
+            _native?.Dispose();
             _native = null;
         }
     }