Sometimes user_info comes as None, and do_shell fails to execute

I was testing the "will sleep" event with the default configuration file that comes with crankd and on my macOS 10.13.6 system, user_info comes as None, which implies it doesn't have the items property and it, therefore, interrupts crankd execution

https://github.com/MacSysadmin/pymacadmin/blob/892f3b56883c9eb1e9f50432cbc8c9a41db7350f/bin/crankd.py#L592