Allow search summary to be generated when a value is missing from the rules.
Allow search summary to be generated when a value is missing from the rules.

 - previously, non-null but incomplete filter rules would have caused a
   crash. Now, the missing values are treated the same as when there are
   no filter-specific rules, i.e. no additional preposition is added.

https://trello.com/c/3XUKd0tX/345-add-filters-to-search-results-page

alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend