Dealing with hook_cron when configuration is required
If you require configuration, then you shouldn't provide a default value using variable get (You provide an empty default value instead). If the no value is set, you should make a log using watchdog and exist without doing anything, since the conf was missing.

If you can provide a default settings that would make everything work, that would be a better option, since the user don't have to do any setup, be can if he wants to.

The checks should not really happen in cron, but in the function actually doing whatever needs to be done.