test(docs-infra): ignore empty lines in `URLS_TO_REDIRECT.txt` (#41680)

Previously, the `URLS_TO_REDIRECT.txt` file was expected to not contain
any empty lines. This could easily result in test errors when updating
the file, since it is common for IDEs/editors to automatically ensure
there is an empty line at the end of a saved file
([example failure][1]).

This commit updates the test helpers to be able to cope with empty or
whitespace-only lines in `URLS_TO_REDIRECT.txt` by ignoring such lines.

[1]: https://circleci.com/gh/angular/angular/965534

PR Close #41680

aio/tests/deployment/shared/URLS_TO_REDIRECT.txt

@@ -194,4 +194,4 @@
 /start/forms	/start/start-forms
 /start/routing	/start/start-routing
 /testing	/guide/testing
-/testing/first-app-tests.html	/guide/testing
+/testing/first-app-tests.html	/guide/testing

aio/tests/deployment/shared/helpers.ts

@@ -41,7 +41,10 @@ export function loadRedirects(): FirebaseRedirectConfig[] {
 
 export function loadLegacyUrls() {
   const pathToLegacyUrls = `${__dirname}/URLS_TO_REDIRECT.txt`;
-  const urls = readFileSync(pathToLegacyUrls, 'utf8').split('\n').map(line => line.split('\t'));
+  const urls = readFileSync(pathToLegacyUrls, 'utf8')
+      .split('\n')
+      .filter(line => line.trim() !== '')
+      .map(line => line.split('\t'));
   return urls;
 }