get rid of the "undefined" message

Hi, 
Whenever one of the "source" divs is not present, it will display "undefined".

my situation: 
source: '#main_nav, #sub_nav'

sometimes there's no #sub_nav, so within the sidr-inner div it will display 'undefined' instead of nothing. 

It would be nice if the plugin could fail more gracefully.  

Cheers
Stefan

btw: I love sidr ;)