GA4: Fix ecommerce tracking of searches without query

The GA4 ecommerce tracking for search results doesn't currently work if
the user is searching for an empty query, as the check for whether the
`ga4-search-query` data attribute is present uses loose Javascript
boolean logic (i.e. returning false for an empty string).

This amends it to strictly check for `null` instead (data attribute not
present at all), and adds some tests for the search ecommerce data being
properly set with the query present and absent.

app/assets/javascripts/govuk_publishing_components/analytics-ga4/ga4-core.js

@@ -333,7 +333,7 @@ window.GOVUK.analyticsGa4 = window.GOVUK.analyticsGa4 || {};
         var element = data.element
         var resultsId = data.resultsId
         var isClickEvent = data.event !== undefined
-        var isSearchResult = element.getAttribute('data-ga4-search-query')
+        var isSearchResult = element.getAttribute('data-ga4-search-query') !== null
 
         var ecommerceSchema = new window.GOVUK.analyticsGa4.Schemas().ecommerceSchema()
         var DEFAULT_LIST_TITLE = 'Smart answer results'

spec/javascripts/govuk_publishing_components/analytics-ga4/ga4-core.spec.js

@@ -629,6 +629,36 @@ describe('GA4 core', function () {
         expect(builtEcommerceObject).toEqual(expectedEcommerceObject)
       })
 
+      it('tracks variant and term for search results', function () {
+        resultsParentEl.setAttribute('data-ga4-search-query', 'search term')
+        resultsParentEl.setAttribute('data-ga4-ecommerce-variant', 'upside-down')
+
+        expectedEcommerceObject.search_results.term = 'search term'
+        expectedEcommerceObject.search_results.sort = 'upside-down'
+
+        var builtEcommerceObject = GOVUK.analyticsGa4.core.ecommerceHelperFunctions.populateEcommerceSchema({
+          element: resultsParentEl,
+          resultsId: 'result-count'
+        })
+
+        expect(builtEcommerceObject).toEqual(expectedEcommerceObject)
+      })
+
+      it('tracks variant and term for search results even when query is blank', function () {
+        resultsParentEl.setAttribute('data-ga4-search-query', '')
+        resultsParentEl.setAttribute('data-ga4-ecommerce-variant', 'upside-down')
+
+        expectedEcommerceObject.search_results.term = undefined
+        expectedEcommerceObject.search_results.sort = 'upside-down'
+
+        var builtEcommerceObject = GOVUK.analyticsGa4.core.ecommerceHelperFunctions.populateEcommerceSchema({
+          element: resultsParentEl,
+          resultsId: 'result-count'
+        })
+
+        expect(builtEcommerceObject).toEqual(expectedEcommerceObject)
+      })
+
       it('the ecommerce items array is limited to a maximum of 15,000 UTF-16 code units', function () {
         var innerHTML = ''
         var ecommerceItems = []