Fix PHP 8.1 "strlen(null)" exceptions which block rendering the DarkConsole

Summary:
Since PHP 8.1 the function call strlen(null) is deprecated.

Since Phabricator/Phorge elevates deprecation warnings to exceptions, the
specific fix implies dealing with null, and dealing with real strings.

In the specific case, a simple cast was made to deal with real strings.

Closes T15315

Test Plan: Visit the Home Page with DarkConsole enabled in your configuration: no crashes

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: aklapper, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15315

Differential Revision: https://we.phorge.it/D25165