Fix PHP 8.1 "strlen(null)" exception which blocks rendering "Browse Dashboards" dialog

Summary:
strlen() was used in Phabricator to check if a generic value is a non-empty string.

But, that function does not accept null anymore since PHP 8.1.

Closes T15396

Test Plan: Applied this change (on top of D25179 and D25226) and the "Browse Dashboards" dialog got rendered in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15396

Differential Revision: https://we.phorge.it/D25227