[XAML] Fix for exception thrown in WholeItemsPanel

[paulcam206]

Looks like a simple mistake in the conversion to cppwinrt (as vs. try_as). The converted version would throw instead of quietly skipping the check...

original

AdaptiveCards/source/uwp/Renderer/lib/WholeItemsPanel.cpp

Lines 469 to 478 in d608411

	if (SUCCEEDED(spChild.As(&spWholeItemPanel)))
	{
	RETURN_IF_FAILED(spWholeItemPanel->IsTruncated(&isChildTruncated));
	if (isChildTruncated)
	{
	// No need to continue the iteration
	*childTruncated = true;
	return S_OK;
	}
	}

converted

AdaptiveCards/source/uwp/Renderer/lib/WholeItemsPanel.cpp

Lines 357 to 363 in 1dcef8c

	if (auto childAsWholeItemPanel = child.as<winrt::WholeItemsPanel>())
	{
	if (childAsWholeItemPanel.IsTruncated())
	{
	return true;
	}
	}

[beervoley]

#8841 - this will have to go in before this PR :)