WebSession: fix crash when no admin user exists
WebSession: fix crash when no admin user exists

* When no user nick-named admin existed, and another user wanted to access
  Personalize menu, this has led to Internal Server Error crash due to
  non-handled exceptional situation.  This patch fixes the problem.

* Cherry-picked from 0935b38cadaf0baa825dc0d1c51e422fcf89a2c7.

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