Update 'no results' partial

Why these changes are being introduced:

When a search returns no results, the filter sidebar should be
omitted and the 'no results' text should be larger. This change
was requested as part of the GDT project, but it should apply to
all TIMDEX UI apps.

Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/GDT-133
* https://mitlibraries.atlassian.net/browse/GDT-161

How this addresses that need:

Updates the 'no results' text and implements the requested styling
changes. The filter logic changes a bit to hide the sidebar:

* In the search controller, empty aggregations are no longer
extracted from the response.
* In the results view, a few conditionals have been added to hide
the filter sidebar, omit the `layout1q3q` class from the results
wrapper, and render the 'no results' message as a header rather
than a list element.
* If a search returns no results, the pagination partial is
hidden.

Side effects of this change:

* The results view is somewhat less elegant as a result of these
changes.
* A guard clause has been added to `SearchController#extract_filters`
to protect against calling `select` on `nil`. (This can occur if
the API returns errors, in which case it will return no aggregations.)
* The `result_empty` partial has been removed. It is only one line
of HTML that feels more readable inline.
* Development and review of this work revealed accessibility concerns
about the results summary (including the 'no results' message) being
too deep in the tab order. This information should be foregrounded
for screen reader users. I've opened GDT-161 to address this.

Author: jazairi

app/controllers/search_controller.rb

@@ -18,7 +18,7 @@ def results
     # The @pagination instance variable includes info about next/previous pages (where they exist) to assist the UI.
     @pagination = Analyzer.new(@enhanced_query, response).pagination if @errors.nil?
 
-    # Display stuff
+    # Display results
     @results = extract_results(response)
     @filters = extract_filters(response)
   end
@@ -30,7 +30,10 @@ def extract_errors(response)
   end
 
   def extract_filters(response)
-    response&.data&.search&.to_h&.dig('aggregations')
+    aggs = response&.data&.search&.to_h&.dig('aggregations')
+    return if aggs.blank?
+
+    aggs.select { |_, agg_values| agg_values.present? }
   end
 
   def extract_results(response)

app/views/search/_result_empty.html.erb

@@ -48,24 +48,34 @@
 
   <%= render(partial: 'shared/error', collection: @errors) %>
 
-  <div class="layout-1q3q layout-band top-space">
+  <div class="<%= 'layout-1q3q' if @filters.present? %> layout-band top-space">
     <div class="col3q wrap-results">
-      <ul id="results" class="list-unbulleted">
-        <%= render(partial: 'search/result', collection: @results) || render('result_empty') %>
-      </ul>
+      <% if @results.present? %>
+        <ul id="results" class="list-unbulleted">
+          <%= render(partial: 'search/result', collection: @results) %>
+        </ul>
+      <% else %>
+        <div id="results">
+          <p class="hd-2">No results found for your search</p>
+        </div>
+      <% end %>
     </div>
 
-    <aside class="col1q filter-container">
-      <div id="filters">
-        <h2>Available filters</h2>
-        <% @filters&.each do |category, values| %>
-          <%= render(partial: 'search/filter', locals: {category: category, values: values}) %>
-        <% end %>
-      </div>
-    </aside>
+    <% if @filters.present? %>
+      <aside class="col1q filter-container">
+        <div id="filters">
+          <h2>Available filters</h2>
+          <% @filters&.each do |category, values| %>
+            <%= render(partial: 'search/filter', locals: {category: category, values: values}) %>
+          <% end %>
+        </div>
+      </aside>
+    <% end %>
   </div>
 
-  <div id="pagination">  
-    <%= render partial: "pagination" %>
-  </div>
+  <% if @results.present? %>
+    <div id="pagination">  
+      <%= render partial: "pagination" %>
+    </div>
+  <% end %>
 </div>

app/views/search/results.html.erb

@@ -201,13 +201,20 @@ class SearchControllerTest < ActionDispatch::IntegrationTest
                      match_requests_on: %i[method uri body]) do
       get '/results?q=asdfiouwenlasd'
       assert_response :success
+
       # Result list contents state "no results"
       assert_select '#results'
       assert_select '#results', { count: 1 }
-      assert_select '#results li', 'There are no results.'
+      assert_select '#results p', 'No results found for your search'
+
+      # Filter sidebar is not shown
+      assert_select '#filters', { count: 0 }
+
       # Filters are not shown
-      assert_select '#filters'
       assert_select '#filters .category h3', { count: 0 }
+
+      # Pagination is not shown
+      assert_select '#pagination', { count: 0 }
     end
   end