build(aio): fix warning about missing <h1>

Fixes #17549

aio/tools/transforms/angular-base-package/post-processors/h1-checker.js

@@ -34,5 +34,5 @@ function getText(h1) {
     (node.properties.ariaHidden === 'true' || node.properties['aria-hidden'] === 'true')
   ));
 
-  return toString(cleaned);
-}
+  return cleaned ? toString(cleaned) : '';
+}

aio/tools/transforms/angular-base-package/post-processors/h1-checker.spec.js

@@ -69,4 +69,14 @@ describe('h1Checker postprocessor', () => {
     processor.$process([doc]);
     expect(doc.vFile.title).toEqual('What is Angular?');
   });
-});
+
+  it('should not break if the h1 is empty (except for an aria-hidden anchor)', () => {
+    const doc = {
+      docType: 'a',
+      renderedContent: `
+        <h1><a aria-hidden="true"></a></h1>
+      `
+    };
+    expect(() => processor.$process([doc])).not.toThrow();
+  });
+});