Prevent exceptions when removing styles.

When `_target.Owner` was null, the resource observable produced a `null` which may not be valid for the target property, resulting in an exception . Although the exception is caught, it slows things down.

src/Avalonia.Styling/Controls/ResourceNodeExtensions.cs

@@ -147,13 +147,16 @@ namespace Avalonia.Controls
             {
                 if (_target.Owner is object)
                 {
-                    observer.OnNext(Convert(_target.Owner?.FindResource(_key)));
+                    observer.OnNext(Convert(_target.Owner.FindResource(_key)));
                 }
             }
 
             private void PublishNext()
             {
-                PublishNext(Convert(_target.Owner?.FindResource(_key)));
+                if (_target.Owner is object)
+                {
+                    PublishNext(Convert(_target.Owner.FindResource(_key)));
+                }
             }
 
             private void OwnerChanged(object sender, EventArgs e)