WebSearch: fix for empty coathors
WebSearch: fix for empty coathors

* When summarising co-authors for an empty author name, get_coauthors()
  was entering runaway loop, leading to intbitset iteration exception.
  This patch fixes the problem.  Thanks to Benoit Thiell for reporting
  the problem.

tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio