R1:10fae71a042f

Fix PHP 8.1 "explode(null)" exception which blocks rendering "Provide Multi-Factor Credentials" page

Summary:
Passing null to the `$string` in `explode(string $separator, string $string, int $limit)` is deprecated since PHP 8.1.
Thus do not explode but create an empty array if the input is null.

Closes T15397

Test Plan: Applied this change and afterwards the "Provide Multi-Factor Credentials" page at `/auth/finish/` showing the "App Code" field correctly rendered in the web browser.

Reviewers: O1…