WIP lxml none value fix for websearch external collections parser

This test is being done six times in this PR.  Cannot we simply move it to `configure-tests.py` so that people installing Invenio would be stopped when they use older version?  (Like we do for MySQLdb version check.)  BTW lxml 2.0.11 was released in 2008, so chances are earlier versions won't be found on many installations anymore...