Change options_html to return <option> for 1-item groups

Previous versions of Choices.js happily accepted HTML like this:

```html
<optgroup label="">
  <option value="">All organisations</option>
</optgroup>
```

However, in v11+ of Choices.js, that's no longer allowed -
presumably since this change:
Choices-js/Choices#615

The option is now silently dropped from rendering. Choices.js now
seems to expect such HTML to be passed outside of an `<optgroup>`,
i.e. `<option value="">All organisations</option>`.

NB, it is perfectly fine to mix top-level `<option>`s with
`<optgroup>`s:
https://www.w3.org/TR/WCAG20-TECHS/H85.html#:~:text=The%20optgroup%20element%20should%20be,desired%20intent%20when%20using%20this.

This work was made a bit tricky by us having separate `options`
and `grouped_options` parameters, meaning we have to maintain
two separate methods, making behaviour change more difficult.
I had hoped to consolidate onto one single `options` parameter
and have the helper be clever enough to dynamically return
options or optgroups, but in my attempt in 2b643a8
there were so many test failures that this wider rearchitecture
was not worth the time.

lib/govuk_publishing_components/presenters/select_with_search_helper.rb

@@ -35,10 +35,7 @@ def css_classes
       def options_html
         if @grouped_options.present?
           blank_option_if_include_blank +
-            grouped_options_for_select(
-              transform_grouped_options(@grouped_options),
-              selected_option,
-            )
+            grouped_and_ungrouped_options_for_select(@grouped_options)
         elsif @options.present?
           blank_option_if_include_blank +
             options_for_select(
@@ -54,6 +51,24 @@ def data_attributes
         }.compact
       end
 
+      def grouped_and_ungrouped_options_for_select(unsorted_options)
+        # Filter out the 'single option' options and treat them as simply `<option>`
+        # The remainder should be treated as true 'grouped options', i.e. `<optgroup>`
+        single_options = []
+        grouped_options = []
+        unsorted_options.each_with_index do |(group, options), _index|
+          if group == ""
+            single_options << options
+          else
+            grouped_options << [group, options]
+          end
+        end
+        single_options.flatten!
+
+        options_for_select(transform_options(single_options), selected_option) +
+          grouped_options_for_select(transform_grouped_options(grouped_options), selected_option)
+      end
+
     private
 
       def transform_options(options)

test/unit/lib/govuk_publishing_components/presenters/select_with_search_helper_test.rb

@@ -68,14 +68,8 @@ class SelectWithSearchHelperPresenterTest < PresenterTestCase
     )
 
     expected = <<~HTML.strip
-      <optgroup label="">
       <option selected="selected" value="">All organisations</option>
-      </optgroup>
-
-      <optgroup label="">
       <option value="foo">All foo</option>
-      </optgroup>
-
       <optgroup label="Live organisations">
       <option value="foo">foo</option>
       <option value="bar">bar</option>