fix php 8.5 deprecated message

[masteradhoc]

Fixes #761

What?

Fixes a PHP deprecation warning caused by using null as an array offset when determining the primary two-factor provider.

Why?

PHP 7.4+ emits a deprecation notice when null is used as an array key. This can happen if no primary provider is selected for a user, resulting in noisy logs and potential issues on stricter setups.

How?

Explicitly guard against null (or empty string) before using it as an array offset.

Testing Instructions

1. setup totp, backup codes and email codes for a user
2. go to /wp-admin/users.php
3. see the deprecation message
4. apply fix
5. go to /wp-admin/users.php - no deprecation message anymore showing up

Screenshots or screencast

Changelog Entry

Fixed - Prevent PHP 7.4+ deprecation warning when resolving the primary two-factor provider.

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