Fix CI build errors

Update Label_Loaded method with nullability and logic

The `Label_Loaded` method in `ButtonPopup.xaml.cs` was updated to
accept a nullable `sender` parameter by changing its signature
to `void Label_Loaded(object? sender, EventArgs e)`.

Additionally, the method implementation was added to check if
the `sender` is of type `Label`. If so, it casts the `sender`
to a `Label` and calls the `SetSemanticFocus` method on it.

Author: IeuanWalker

samples/CommunityToolkit.Maui.Sample/Views/Popups/ButtonPopup.xaml.cs

@@ -12,7 +12,7 @@ void Button_Clicked(object? sender, EventArgs e)
 		CloseAsync();
 	}
 
-	void Label_Loaded(object sender, EventArgs e)
+	void Label_Loaded(object? sender, EventArgs e)
 	{
 		if (sender is Label label)
 		{