Do not evaluate NULL email addresses

What the php version and drupal?

This happens because for some reason, _unwanted_email_registration_filter_email() is called twice. The first time $email is properly populated, the second time it is NULL.

PHP 8.2, Drupal 10.2.3.

Previous patch seems not fixing the problem.

The problem is that code doesn't takes into account email with NULL value (for example Anonymous user with uid == 0)

The MR below skips empty email values and continues the loop to check the rest emails:
https://git.drupalcode.org/project/unwanted_email_registration/-/merge_r...

It's a really small but working change, so if no one else has any objection, I set it as RTBC.