Fix PHP 8.1 substr(null) in People > Activity Logs
When someone tries to log on with invalid credentials, a record is inserted in the phabricator_user.user_log table with action = login-fail.
The session value of this record is set to null in this case.
When you open the People → Activity Logs screen, you will see an exception because of this.
Test Plan: Go to https://my.phorge.site/people/logs/ after a login failure and a login success.
Reviewers: O1 Blessed Committers, avivey
Reviewed By: O1 Blessed Committers, avivey
Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15555
Differential Revision: https://we.phorge.it/D25351