Prevent trying to destroy base black texture from test result window

Tests/Scripts/Editor/GraphicTests/Framework/TestResultWindow.cs

         private void OnDisable()
         {
             DestroyImmediate(templateImage);
-            DestroyImmediate(resultImage);
+            if(resultImage != null && resultImage != Texture2D.blackTexture) DestroyImmediate(resultImage);
             DestroyImmediate(diffMaterial);
         }