Add null conditional operator into SwipeItemView OnCommandCanExecuteChanged method (#15869)

Co-authored-by: Federico Nembrini <25205086+FedericoNembrini@users.noreply.github.com>

Author: jfversluis

Xamarin.Forms.Core/SwipeItemView.cs

@@ -64,7 +64,7 @@ void OnCommandParameterChanged()
 
 		void OnCommandCanExecuteChanged(object sender, EventArgs eventArgs)
 		{
-			IsEnabled = Command.CanExecute(CommandParameter);
+			IsEnabled = Command?.CanExecute(CommandParameter) ?? true;
 		}
 	}
-}
+}