Better support for empty elements in manifest.

[dimarobert]

Fixes #7301

Summary

Adds support for both forms of empty elements in manifest.

As the documentation states, System.Xml.XmlReader.IsEmptyElement supports only self-closing elements:

gets a value indicating whether the current node is an empty element (for example, <MyElement/>).

Returns
   true if the current node is an element (NodeType equals XmlNodeType.Element) that ends with />; otherwise, false.

[bdukes]

Thanks! I pushed a couple of commits to add on to your changes.

1. I added some tests to verify what should be happening here.
2. I added a helper XmlUtils.TryReadEndElement, which also handles an element which has only whitespace.