fix(job): set default XXL-Job access token

pigx-common/pigx-common-job/src/main/java/com/pig4cloud/pigx/common/job/properties/XxlJobProperties.java

@@ -17,7 +17,7 @@ public class XxlJobProperties {
     /**
      * 执行器通讯TOKEN [选填]：非空时启用；
      */
-    private String accessToken;
+    private String accessToken = "default_token";
 
     @NestedConfigurationProperty
     private XxlAdminProperties admin = new XxlAdminProperties();