Added function to consistently check empty variables
It turned out that in special circumstances Actions must check for several values to "decide" whether a variable is empty or not. This should be more easy and consistent.
So with the new function `checkEmpty` it is now possible to consistently check for variables being (or behaving) empty in a quite simple way.
This mainly helps avoiding boolean false checks on empty variables returning true.
Help yourself and refactor your Actions to use `checkEmpty` before checking against types and values.