[fix] #442: condition to check if block exist failed: - block root '0' was making the ckech fail. - Check variable if it's none.
[fix] #442: condition to check if block exist failed:
- block root '0' was making the ckech fail.
- Check variable if it's none.

ucoin-io/cutecoin,ucoin-io/cutecoin,ucoin-io/cutecoin