Avoid undefined method errors when no sources have been added

Hi,

Following your docs, on RHEL 6, Chef 10.14.4 (embedded ruby 1.9.1), without configuring any sources, I got undefined method errors due to node.run_state[:sumo_source] being nil.  My fix was to make sure that node.run_state[:sumo_output] == [] and then skipping the original assignment in that case.

-Bryce.