Package.check always return check_delta time

Fix return NoneType in check:
[INFO] imapsync: Next check is planned in 3358s
[CRITICAL] '<' not supported between instances of 'int' and 'NoneType'
Traceback (most recent call last):
  File "/usr/bin/aurbot", line 458, in <module>
    Robot().start()
  File "/usr/bin/aurbot", line 449, in start
    timeout = min(next_checks)
TypeError: '<' not supported between instances of 'int' and 'NoneType'