Deal better with empty MAC addresses

Agents on Windows soemtimes include an empty MAC address in the list.
This is a simple workaround to deal with that.

Of course, the agent should not be sending values like that in the first
place.
Simplify turning all mac addresses to lower case
Move checkInteger code inline

The checkInteger method was short and used only in one place.
Work on mac_addresses only if it is not None