Improve return type of getPageTitle test helper

addon/src/test-support/get-page-title.ts

@@ -1,17 +1,17 @@
 // Testem appends progress to the title...
 // and there's no way to stop this at the moment
 
-export function getPageTitle(doc: Document) {
+export function getPageTitle(doc?: Document): string {
   // In Fastboot context we get 2 title elements if we don't remove one from app/index.html
   // In real world applications, it is mandatory to remove <title> from app/index.html
   // We are keeping both for sake for testing browser and fastboot scenarios
   const element = [
     ...(doc || window.document).querySelectorAll('head title'),
   ].pop();
 
-  return (
-    element &&
-    element instanceof HTMLTitleElement &&
-    element.innerText.trim().replace(/^\(\d+\/\d+\)/, '')
-  );
+  if (element instanceof HTMLTitleElement) {
+    return element.innerText.trim().replace(/^\(\d+\/\d+\)/, '');
+  }
+
+  return '';
 }