PHP 8.1: Fix addslashes(): Passing null to parameter #1 () of type st…

…ring is deprecated (#140)

* PHP 8.1: Fix addslashes(): Passing null to parameter #1 () of type string is deprecated

* Fix linting since it complains about the null coalescing

* PHP 8.1: Fix addslashes(): Passing null to parameter #1 () of type string is deprecated

db.php

@@ -841,7 +841,7 @@ public function set_charset( $dbh, $charset = null, $collate = null ) {
 	 * This is also the reason why we don't allow certain charsets. See set_charset().
 	 */
 	public function _real_escape( $string ) {   // phpcs:ignore PSR2.Methods.MethodDeclaration.Underscore
-		$escaped = addslashes( $string );
+		$escaped = addslashes( (string) $string );
 		if ( method_exists( get_parent_class( $this ), 'add_placeholder_escape' ) ) {
 			$escaped = $this->add_placeholder_escape( $escaped );
 		}