Decoding null to null is not an error

Quit throwing exceptions when the queue is empty.

Change-Id: Ie5479c81c0a6138e1cad416b40d3edabffb1e9c4

Author: ejegg

src/PHPQueue/Json.php

@@ -5,6 +5,9 @@
 
 class Json {
     public static function safe_decode( $text ) {
+        if ( $text === null ) {
+            return null;
+        }
         $data = json_decode($text, true);
         if ( $data === null ) {
             throw new JsonException("JSON could not be decoded: '{$text}'");