Using transparent brush when image source is null

src/Skia/Avalonia.Skia/DrawingContextImpl.cs

@@ -491,6 +491,10 @@ namespace Avalonia.Skia
             {
                 ConfigureTileBrush(ref paintWrapper, targetSize, tileBrush, tileBrushImage);
             }
+            else
+            {
+                paint.Color = new SKColor(255, 255, 255, 0);
+            }
 
             return paintWrapper;
         }

tests/Avalonia.RenderTests/Media/ImageBrushTests.cs

@@ -47,6 +47,7 @@ namespace Avalonia.Direct2D1.RenderTests.Media
             };
 
             await RenderToFile(target);
+            CompareImages();
         }
 
         [Fact]