WKWebView in ASDisplayNode doesn't work with VoiceOver #1954

If a WKWebView is loaded into an ASDisplayNode, the web view's HTML contents are not accessible to VoiceOver. I will upload a sample project later, or if I can get the test target building on Xcode 12, I'll write a unit test that demonstrates the problem.

A potential fix that seems to work: modify _ASDisplayViewAccessibility.mm's implementation of -accessibilityElements to return nil instead of an empty array if it does not find any elements. I believe this is because WKWebView's accessibility elements are handled out-of-process. By returning an empty array, I think we were telling the system that there were no accessibility elements.

Reproduced on iOS 14.3.