[iOS] Avoid non-`UIElement` items in `ChildrenShadow`

[MartinZikmund]

What would you like to be added

In some cases, iOS adds custom temporary UIViews (accessibility/input related). as children of our UIElements. This is a problem as Children as a UIElementCollection assumes that there will be no null items. Hence we need to make sure to filter away the non-UIElement children and avoid their projection to Children as null

Why is this needed

Reliability.

For which platform

iOS

Anything else we need to know?

No response

[Youssef1313]

NOTE: When fixing this, the workaround in https://github.com/unoplatform/uno/pull/18470 can be reverted