Fix crash when enumerating lights that don't have a brightness atrribute

[majick]

Some devices do not have a brightness, such as switches:

$ hue 23 dump   # A switched outlet
{"on":true,"alert":"none","mode":"homeautomation","reachable":true}

$ hue 2 dump  # A light
{"on":false,"bri":76,"ct":367,"alert":"none","colormode":"ct","mode":"homeautomation","reachable":true}

$ hue | egrep '( 2\.|23\.)'
     2. D1 Bulb                          OFF     LTW011 - Color temperature light
    23. Fish Light                       ON      LXN56-0S27LX1.1 - On/Off light

This causes the hue lights command to crash when trying to access the brightness method. This small change prevents the crash and allows hue lights to continue.

[majick]

This addresses #10