code: def _process_filters(filter_dictionary):
      
      def filter_item(field):
          """ format elasticsearch filter to pass if value matches OR field is not included """
          if filter_dictionary[field] is not None:
              return {
                  "or": [
                      _get_filter_field(field, filter_dictionary[field]),
                      {
                          "missing": {
                              "field": field
                          }
                      }
                  ]
              }
  
          return {
              "missing": {
                  "field": field
              }
          }
  
      return [filter_item(field) for field in filter_dictionary]
docs: We have a filter_dictionary - this means that if the field is included
      and matches, then we can include, OR if the field is undefined, then we
      assume it is safe to include