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.

Signed-off-by: Sebastian Witowski <sebastian.witowski@cern.ch>
Merge pull request #40 from switowski/search_engine_None_in_intbitset_fix

Websearch: fix for 'rhs is of unknown type'