455485: xslfo link handling issues

XslfoDocumentBuilder does not handle links correctly:
* hyperlinks without label are not handled as links at all
* external links duplicated.
This patch fixes the code and adds a test for this issue.

Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=455485
Change-Id: I1307de30e1526beffafa6de9b333fe4f84bfb01a
Signed-off-by: Ulf Winkelvos <ulf.winkelvos@mgm-tp.com>