Filter GetVisualTreeDescendants to not check elements with null handlers

[drasticactions]

Description of Change

Filter VisualElements that don't have handlers set from being returned inside GetVisualTreeDescendants. Calls to this method that contain elements that don't have handlers set would throw Handler exceptions as it's null.

Issues Fixed

Fixes #16919

[drasticactions]

We're still talking about the best ways to handle StrokeShape. IMO this is more of a "Workaround" for the general issue linked above. By this point in the views lifecycle, they should have a handler (You invoke these APIs by giving an x, y target or by calling an existing IVisualTreeElement, which should be an element that exists and is visible in the visual tree). The issue are some elements like Border.StrokeShape that use elements that "do" have handlers, but are not set and they are not directly drawn on UI.