Safeguard ErrorSummary instantiation without an element

This complements the guard within our `initAll` function. It'll protect
users code when:
- they would manually initialise an `ErrorSummary`
- but sometimes not pass an element (because there is none collected
  by a `document.querySelector`, for example).
That'll avoid the JavaScript execution stopping because of the missing element
and their code will carry on running as it does now.

src/govuk/components/error-summary/error-summary.mjs

@@ -15,6 +15,19 @@ import { mergeConfigs, normaliseDataset } from '../../common.mjs'
  * @param {Boolean} [config.disableAutoFocus=false] - Whether to disable the component taking focus on initialisation
  */
 function ErrorSummary ($module, config) {
+  // Some consuming code may not be passing a module,
+  // for example if they initialise the component
+  // on their own by directly passing the result
+  // of `document.querySelector`.
+  // To avoid breaking further JavaScript initialisation
+  // we need to safeguard against this so things keep
+  // working the same now we read the elements data attributes
+  if (!$module) {
+    // Little safety in case code gets ported as-is
+    // into and ES6 class constructor, where the return value matters
+    return this
+  }
+
   this.$module = $module
 
   var defaultConfig = {

src/govuk/components/error-summary/error-summary.test.js

@@ -88,6 +88,21 @@ describe('Error Summary', () => {
       })
     })
 
+    describe('using JavaScript configuration, with no elements on the page', () => {
+      it('does not prevent further JavaScript from running', async () => {
+        const result = await page.evaluate(() => {
+          // `undefined` simulates the element being missing,
+          // from an unchecked `document.querySelector` for example
+          new window.GOVUKFrontend.ErrorSummary(undefined).init()
+
+          // If our component initialisation breaks, this won't run
+          return true
+        })
+
+        expect(result).toBe(true)
+      })
+    })
+
     describe('using JavaScript configuration, but enabled via data-attributes', () => {
       beforeAll(async () => {
         await renderAndInitialise('error-summary', {