Ensuring that CatalogController utilizes the non-default SearchBuilder

when configured for a custom SearchBuilder and the `search_algorith`
parameter is empty

Co-authored-by: Anna Headley <hackartisan@users.noreply.github.com>
Co-authored-by: Carolyn Cole <carolyncole@users.noreply.github.com>
Co-authored-by: Trey Pendragon <tpendragon@users.noreply.github.com>
Co-authored-by: Eliot Jordan <eliotjordan@users.noreply.github.com>

app/controllers/catalog_controller.rb

@@ -794,8 +794,14 @@ def search_service_context
     { search_builder_class: alternate_search_builder_class }
   end
 
+  def search_algorithm_param
+    params[:search_algorithm]
+  end
+
   def alternate_search_builder_class
-    "#{params[:search_algorithm]}_search_builder".camelize.constantize
+    return unless search_algorithm_param
+
+    "#{search_algorithm_param}_search_builder".camelize.constantize
   rescue NameError
     nil
   end

spec/controllers/catalog_controller_spec.rb

@@ -154,6 +154,16 @@
           expect(repository).to have_received(:search).with(instance_of(SearchBuilder))
         end
       end
+
+      context "when the default is not search builder and the search_algorithm parameter is empty" do
+        it "uses the configured search builder" do
+          allow(controller.blacklight_config).to receive(:search_builder_class).and_return(EngineeringSearchBuilder)
+
+          get :index, params: { q: "coffee" }
+
+          expect(repository).to have_received(:search).with(instance_of(EngineeringSearchBuilder))
+        end
+      end
     end
 
     context "when the multi-algorithm feature is off" do