* `illegal_names`
* `limited_email_domains`
* `banned_email_domains`
If any of these options have a falsey value, treat it as an empty string. This addresses a PHP 8.1+ deprecation notice when passing a `null` value to `str_replace()`.
Additionally, avoid unnecessary type casting for better performance.
Props hellofromTonya, jrf, SergeyBiryukov.
See #51423.
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