Don't draw null geometry in GeometryDrawing.

And add a guard to `DrawingContext` to detect having a null `Geometry` passed, as we do for `DrawImage`, `DrawText` etc.

Fixes #2053.

src/Avalonia.Visuals/Media/DrawingContext.cs

@@ -109,6 +109,8 @@ namespace Avalonia.Media
         /// <param name="geometry">The geometry.</param>
         public void DrawGeometry(IBrush brush, Pen pen, Geometry geometry)
         {
+            Contract.Requires<ArgumentNullException>(geometry != null);
+
             if (brush != null || PenIsVisible(pen))
             {
                 PlatformImpl.DrawGeometry(brush, pen, geometry.PlatformImpl);

src/Avalonia.Visuals/Media/GeometryDrawing.cs

@@ -31,7 +31,10 @@
 
         public override void Draw(DrawingContext context)
         {
-            context.DrawGeometry(Brush, Pen, Geometry);
+            if (Geometry != null)
+            {
+                context.DrawGeometry(Brush, Pen, Geometry);
+            }
         }
 
         public override Rect GetBounds()
@@ -41,4 +44,4 @@
             return Geometry?.GetRenderBounds(pen) ?? new Rect();
         }
     }
-}
+}