Add check for customized state path existence before adding listener to it

[zhangmeng916]

Helix has implemented the feature for customized view aggregation. During participant starts up, the path of "customized state" will be created under the instance node. However, many customers are still with old helix-lib, and won't have this path created. If we add listener without checking the existence of this path, there are some exception log thrown by zkclient for no node, and it may confuse the customer.