454822: Avoid NPE when emmitting Conluence link with no href

Change-Id: I66c8c5b697ea66c454243d19f2865124890cd204
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=454822
(cherry picked from commit 927c0b039ebc9f04c2590b31d0ad5096837ba23f)