WebSearch: fix for double display of the footer

• Raises an exception in the case there are no results to display in order to avoid displaying twice the footer and the box proposing links to other useful search engines.

• Removes calls to page_end() and perform_external_collection_search_with_em() when there are no results to display since these functions will be executed in the top level.

• Handles the exception to postpone the execution of those functions.

Co-authored-by: Tibor Simko <tibor.simko@cern.ch>