Fix PHP 8.1 "strlen(null)" and "explode()" exceptions which block rendering Administrator Account Creation page

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts `phutil_nonempty_string()` as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

Similarly, `explode(string $separator, string $string, int $limit)` does not accept
passing null instead of an actual string as input parameter either anymore.

Closes T15284

Test Plan: Applied these two changes. Afterwards, admin user account was created and Phorge homepage rendered in web browser on a fresh installation.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15284

Differential Revision: https://we.phorge.it/D25175

src/applications/auth/constants/PhabricatorCookies.php

@@ -164,7 +164,7 @@ final class PhabricatorCookies extends Phobject {
     // Old cookies look like: /uri
     // New cookies look like: timestamp,/uri
 
-    if (!strlen($cookie)) {
+    if (!phutil_nonempty_string($cookie)) {
       return null;
     }
 

src/applications/people/cache/PhabricatorUserProfileImageCacheType.php

@@ -91,6 +91,9 @@ final class PhabricatorUserProfileImageCacheType
   }
 
   public function isRawCacheDataValid(PhabricatorUser $user, $key, $data) {
+    if ($data === null) {
+      return false;
+    }
     $parts = explode(',', $data, 2);
     $version = reset($parts);
     return ($version === $this->getCacheVersion($user));