cxmanage: Allow "sensor" command with no sensor_name argument
cxmanage: Allow "sensor" command with no sensor_name argument

In this case it's exactly like "sensor list" -- it'll just display
all of the sensors.

SilverLiningSystems/cxmanage-test2,SilverLiningSystems/cxmanage-test2,SLS-Dev/cxmanage,Cynerva/cxmanage,SilverLiningSystems/cxmanage-test,Cynerva/cxmanage,SilverLiningSystems/cxmanage-test,SLS-Dev/cxmanage