Code Modernization: Return an empty string from wpdb::_real_escape() if a non-scalar value is passed.

This avoids a fatal error on PHP 8 caused by passing a non-string value to ` mysqli_real_escape_string()`, and maintains the current behaviour.

See #50913, #50639.

git-svn-id: https://develop.svn.wordpress.org/trunk@48980 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/wp-db.php

@@ -1159,6 +1159,10 @@ class wpdb {
 	 * @return string Escaped string.
 	 */
 	function _real_escape( $string ) {
+		if ( ! is_scalar( $string ) && ! is_null( $string ) ) {
+			return '';
+		}
+
 		if ( $this->dbh ) {
 			if ( $this->use_mysqli ) {
 				$escaped = mysqli_real_escape_string( $this->dbh, $string );