Code Modernization: Fix null to non-nullable deprecation in `wp_priva…

…cy_anonymize_ip()`.

The `wp_privacy_anonymize_ip()` function expects a string for the `$ip_addr` parameter, but did not do any input validation.

One of the pre-existing test cases, passed `null` to the function, leading to a `substr_count(): Passing null to parameter #1 ($haystack) of type string is deprecated` notice on PHP 8.1.

Fixed now by doing a cursory check on the variable at the start of the function and bowing out early for a number of cases (`null`, `false`, `0`, `''`) which would all result in the same `0.0.0.0` output anyway.

Follow-up [42971].

Props jrf, hellofromTonya.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51793 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -7618,6 +7618,10 @@ function wp_site_admin_email_change_notification( $old_email, $new_email, $optio
  * @return string  The anonymized IP address.
  */
 function wp_privacy_anonymize_ip( $ip_addr, $ipv6_fallback = false ) {
+	if ( empty( $ip_addr ) ) {
+		return '0.0.0.0';
+	}
+
 	// Detect what kind of IP address this is.
 	$ip_prefix = '';
 	$is_ipv6   = substr_count( $ip_addr, ':' ) > 1;