Code Modernization: Fix "passing null to non-nullable" deprecation in wpdb::_real_escape().

The PHP native `mysqli_real_escape_string()` function expects to be passed a string as the second parameter and this is not a nullable parameter.

Passing `null` to it will result in a `mysqli_real_escape_string(): Passing null to parameter #2 ($string) of type string is deprecated` notice on PHP 8.1.

Previously, an input type check was put in place to prevent fatal errors on PHP 8.0 when an array, object or resource was passed. Changeset [48980].

A `null` value was explicitly excluded from that check, even though a `null` value being passed would only ever result in an empty string anyway.

This commit changes the previous input type check to also bow out early for `null` values and to automatically return an empty string for those.

Refs:
- https://www.php.net/manual/en/mysqli.real-escape-string.php
- https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg

Follow-up to [48980].

Props jrf, hellofromTonya.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51799 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/wp-db.php

@@ -1158,7 +1158,7 @@ class wpdb {
 	 * @return string Escaped string.
 	 */
 	function _real_escape( $string ) {
-		if ( ! is_scalar( $string ) && ! is_null( $string ) ) {
+		if ( ! is_scalar( $string ) ) {
 			return '';
 		}