Avoid write null to StateStore

According to get method doc in StateStore API, it returns non-null row if the key exists. So basically we should avoid write null to StateStore. You cannot distinguish if the returned null row is because the key doesn't exist, or the value is actually null. And due to the defined behavior of get, it is quite easy to cause NPE error if the caller doesn't expect to get a null if the caller believes the key exists.