Fix PHP 8.1 substr(null) in People > Activity Logs

Summary:
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.

Fixes T15555

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