R1:f3b37413167e

Fix PHP 8.1 "strlen(null)" exception inCelerityResourceController to make CSS/JS load

Summary:
On a fresh installation with PHP 8.1, CSS does not load on the account creation page.

Closes T15283

Test Plan: After applying this change (and D25137), the account creation page is rendered correctly. See the screenshot in T15283

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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