Fix nil error in confirmation_token url helper

[manno]

  No route matches {:action=>"confirm", :conference_acronym=>"CONFERENCE", :controller=>"cfp/events", :id=>ID, :locale=>"en", :token=>nil}, possible unmatched constraints: [:token]
Did you mean?  cfp_event_confirm_by_token_url
  app/views/cfp/people/_table.html.haml:37

This is related to the partly fix from #817
Conference may have two "event_people" objects for the same "person", one as "submitter", another as "speaker". Only the "speaker" has the confirmation token.