Only show cookie banner if consent required

The cookie banner/option to set cookie preferences will only be used
when attempting to track usage data via Google Tag Manager, at least
initially. The gem already supports the alternative gtag.js approach and
introducing a cookie banner consent step to that flow would be a
breaking change

The conditional used here is similar to one used by the gtag.js partial

Author: yndajas

lib/source/layouts/_cookie_banner.erb

@@ -1,22 +1,24 @@
-<div class="govuk-cookie-banner" data-nosnippet role="region" aria-label="Cookies on <%= config[:tech_docs][:service_name] %>">
-  <div class="govuk-cookie-banner__message govuk-width-container">
-    <div class="govuk-grid-row">
-      <div class="govuk-grid-column-two-thirds">
-        <h2 class="govuk-cookie-banner__heading govuk-heading-m">
-          Cookies on <%= config[:tech_docs][:service_name] %>
-        </h2>
-        <div class="govuk-cookie-banner__content">
-          <p class="govuk-body">We’d like to use analytics cookies so we can understand how you use the service and make improvements.</p>
+<% if config[:tech_docs][:ga4_gtm_tracking_id].is_a?(String) && !config[:tech_docs][:ga4_gtm_tracking_id].empty? %>
+  <div class="govuk-cookie-banner" data-nosnippet role="region" aria-label="Cookies on <%= config[:tech_docs][:service_name] %>">
+    <div class="govuk-cookie-banner__message govuk-width-container">
+      <div class="govuk-grid-row">
+        <div class="govuk-grid-column-two-thirds">
+          <h2 class="govuk-cookie-banner__heading govuk-heading-m">
+            Cookies on <%= config[:tech_docs][:service_name] %>
+          </h2>
+          <div class="govuk-cookie-banner__content">
+            <p class="govuk-body">We’d like to use analytics cookies so we can understand how you use the service and make improvements.</p>
+          </div>
         </div>
       </div>
-    </div>
-    <div class="govuk-button-group">
-      <button type="button" class="govuk-button" data-module="govuk-button">
-        Accept analytics cookies
-      </button>
-      <button type="button" class="govuk-button" data-module="govuk-button">
-        Reject analytics cookies
-      </button>
+      <div class="govuk-button-group">
+        <button type="button" class="govuk-button" data-module="govuk-button">
+          Accept analytics cookies
+        </button>
+        <button type="button" class="govuk-button" data-module="govuk-button">
+          Reject analytics cookies
+        </button>
+      </div>
     </div>
   </div>
-</div>
+<% end %>