critpath: make config value optional
This allows administrators to disable critpath checking by simply not
pointing trigger to any critpath file (the default behavior after this
patch). The job triggers will receive an empty critpath list set in this
case. If a critpath file is set in the config file, the file has to
exist and be valid.
This improves reliability over the previous change in e67e56745ac.
Fixes #54
Merges #56