update document

xxl-job-core/src/main/java/com/xxl/job/core/handler/annotation/XxlJob.java

     /**
      * init handler, invoked when JobThread init
      */
-    String init();
+    String init() default "";
 
     /**
      * destroy handler, invoked when JobThread destroy
      */
-    String destroy();
+    String destroy() default "";
 
 }