WebSearch: fix for 'rhs is of unknown type'
WebSearch: fix for 'rhs is of unknown type'

* Fixes TypeError in search_engine that was occurring when a new
  collection has been added but the webcoll has not yet been run.
  (closes #1819)

Signed-off-by: Sebastian Witowski <f69e5de8dea812f55abb37bf79df36126074c5c0@cern.ch>
Reviewed-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

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