fix(get-ancestry): don't error when there is no parent (#4617)

Seems I removed the conditional check on the children lookup during a
refactor. Adding it back in and ensuring there's a test so this doesn't
happen again.

lib/core/utils/get-ancestry.js

@@ -9,7 +9,7 @@ function generateAncestry(node) {
  if (
  nodeName !== 'head' &&
  nodeName !== 'body' &&
- parentNode.children.length > 1
+ parentNode?.children.length > 1
  ) {
  const index = Array.prototype.indexOf.call(parentNode.children, node) + 1;
  nthChild = `:nth-child(${index})`;

test/core/utils/get-ancestry.js

@@ -26,6 +26,11 @@ describe('axe.utils.getAncestry', () => {
  assert.isNotNull(document.querySelector(sel1));
  });
 
+ it('should not throw when there is no parent', () => {
+ const node = document.createElement('section');
+ assert.doesNotThrow(() => axe.utils.getAncestry(node));
+ });
+
  it('generates selectors of nested shadow trees', () => {
  const node = document.createElement('section');
  fixture.appendChild(node);