Ignore Cisco devices with empty id’s

Dear Egor,

First, let me thank you for your excellent work!

Now I have found that some of my Cisco 892 and 1921 return empty id's for temperature and psu sensors. This patch is maybe not the best way to fix the issue, but it work for me.

Do you have the intention to integrate this, and maybe the work of kb8u's fork and release a new egg soon ? That would be terrific !

Thanks again,
Charles