UIA in MS Word: don't ignore tables with blank cells (#9022)
* UIA WordDocumentTextInfo: remove custom implementation of isCollapsed property. It is no longer needed in recent versions of MS Word, and in fact it was causing tables to not be reported when positioned on a blank cell.
* UIATextInfo.UIAElementAtStart: catch a COMError when fetching children. Seen at the bottom of MS Word documents.
