Fix EmptyStackException thrown by elements with xlink:href attributes and no base_uri.

Nokogiri::XML::Reader required elements with xlink:href attributes to be contained by elements with xml:base attributes.  ReaderNode would throw an EmptyStackException if this wasn't the case.

The xlink spec is ambiguous about whether that situation is allowed, but a few online xlink examples and our real-world data have xlink:href attributes with no xml:base.

Nokogiri uses xlink:href to compute the base_uri attribute, but I didn't see any tests for this, so I wrote two.  The second one fails without my fix.