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