Check if PhabricatorHash::digestForIndex() receives null
Summary:
Do not crash with an error passing down null by introducing a check.
```
ERROR 8192: sha1(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/hosts/__PHABRICATOR__/phabricator/src/infrastructure/util/PhabricatorHash.php:47]
```
Refs T16284
Test Plan: Somehow make `idx($_SERVER, 'REMOTE_ADDR')` return `null`, I guess.
Reviewers: O1 Blessed Committers, avivey
Reviewed By: O1 Blessed Committers, avivey
Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16284
Differential Revision: https://we.phorge.it/D26458