Fix PHP 8.1 "strlen(null)" exceptions which block adding 2FA factor

Summary:
strlen() was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts phutil_nonempty_string() as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

Closes T15388

Test Plan:
Applied these two changes; afterwards managed to add a 2FA factor and /settings/user/username/page/multifactor/?id=1 correctly listed mys Authentication Factors.

Additional tests:

• Unset any eventual personal MFA
• Setup a personal MFA
• Login/Logout using the MFA
• Remove a personal MFA
• Setup a personal enroll message from /auth/mfa/1/
• Setup a personal MFA
• Login/Logout using the MFA
• (then cleanup removing your test MFA)

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15388

Differential Revision: https://we.phorge.it/D25219