Don't create a bitmap if it is going to be zero

source/SkiaSharp.Views/SkiaSharp.Views.UWP/SKXamlCanvas.cs

@@ -156,10 +156,11 @@ namespace SkiaSharp.Views.UWP
 			var size = CreateSize();
 			var info = new SKImageInfo(size.Width, size.Height, SKImageInfo.PlatformColorType, SKAlphaType.Premul);
 
-			if (bitmap == null || bitmap.PixelWidth != info.Width || bitmap.PixelHeight != info.Height)
-			{
-				FreeBitmap();
+            if (bitmap?.PixelWidth != info.Width || bitmap?.PixelHeight != info.Height)
+                FreeBitmap();
 
+            if (bitmap == null && info.Width > 0 && info.Height > 0)
+            {
 				bitmap = new WriteableBitmap(info.Width, info.Height);
 				pixels = bitmap.GetPixels();