Fix PHP 8.1 strlen() error when database replication Seconds_Behind_Master is NULL

Summary:
When MySQL database replication is configured, but not running, the slave status Seconds_Behind_Master is NULL.
When this value is passed to strlen() an error is thrown.

Fixes T15507

Test Plan:
* Configure database replication such that Seconds_Behind_Master is NULL
* Restart php-fpm
* Go to Phorge web site.

Reviewers: O1 Blessed Committers, chris

Reviewed By: O1 Blessed Committers, chris

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

Maniphest Tasks: T15507

Differential Revision: https://we.phorge.it/D25318

src/infrastructure/cluster/PhabricatorDatabaseRef.php

@@ -393,7 +393,7 @@ final class PhabricatorDatabaseRef
 
         if ($is_replica) {
           $latency = idx($replica_status, 'Seconds_Behind_Master');
-          if (!strlen($latency)) {
+          if (!phutil_nonempty_string($latency)) {
             $ref->setReplicaStatus(self::REPLICATION_NOT_REPLICATING);
           } else {
             $latency = (int)$latency;