Safely query Users/Channels/Groups for cache
Safely query Users/Channels/Groups for cache

This commit provides a bit of safety while populating the Slack cache
by providing empty lists for each of the payload queries in the event
it is undefined.

Fixes #180

pearsontechnology/st2contrib,pearsontechnology/st2contrib,tonybaloney/st2contrib,dennybaa/st2contrib,pinterb/st2contrib,armab/st2contrib,pidah/st2contrib,StackStorm/st2contrib,lmEshoo/st2contrib,pidah/st2contrib,psychopenguin/st2contrib,armab/st2contrib,tonybaloney/st2contrib,meirwah/st2contrib,pinterb/st2contrib,meirwah/st2contrib,dennybaa/st2contrib,pearsontechnology/st2contrib,armab/st2contrib,StackStorm/st2contrib,tonybaloney/st2contrib,StackStorm/st2contrib,digideskio/st2contrib,lmEshoo/st2contrib,pidah/st2contrib,digideskio/st2contrib,pearsontechnology/st2contrib,psychopenguin/st2contrib