Handle textbox without existing aria-described-by

jQuery.attr returns `undefined` if an element does not have an
attribute. We want an empty string, rather than the default of coercing
`undefined` to the string `'undefined'`.

app/assets/javascripts/updateStatus.js

@@ -48,7 +48,14 @@
         .attr('id', id);
 
       this.$textbox
-        .attr('aria-described-by', this.$textbox.attr('aria-described-by') + ' ' + id)
+        .attr(
+          'aria-described-by',
+          (
+            this.$textbox.attr('aria-described-by') || ''
+          ) + (
+            this.$textbox.attr('aria-described-by') ? ' ' : ''
+          ) + id
+        )
         .on('input', throttle(this.update, 150))
         .trigger('input');
 

tests/javascripts/updateStatus.test.js

@@ -85,6 +85,20 @@ describe('Update content', () => {
 
   });
 
+  test("It should handle a textarea without an aria-described-by attribute", () => {
+
+    document.getElementById('template_content').removeAttribute('aria-described-by');
+
+    window.GOVUK.modules.start();
+
+    expect(
+      document.getElementById('template_content').getAttribute('aria-described-by')
+    ).toEqual(
+      "update-status"
+    );
+
+  });
+
   test("It should make requests to the URL specified in the data-updates-url attribute", () => {
 
     window.GOVUK.modules.start();