Fix nil error

This fixes an error I had recently where GetSpellCooldown returns nil upon login.

Message: ..\AddOns\Cork\templates\SelfBuffer.lua line 17:
   attempt to perform arithmetic on local 'start' (a nil value)
Debug:
   (tail call): ?
   (tail call): ?
   Cork\templates\SelfBuffer.lua:17:
      Cork\templates\SelfBuffer.lua:13
   Cork\templates\SelfBuffer.lua:27: TestWithoutResting()
   Cork\modules\DarkmoonExp.lua:36: Test()
   Cork\templates\SelfBuffer.lua:65: Scan()
   Cork\Cork.lua:42:
      Cork\Cork.lua:37
   [string "safecall Dispatcher[1]"]:4:
      [string "safecall Dispatcher[1]"]:4
   [C]: ?
   [string "safecall Dispatcher[1]"]:13: ?()
   ...ore\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:90: Fire()
   ...\AddOns\DataStore\libs\AceEvent-3.0\AceEvent-3.0.lua:120:
      ...\AddOns\DataStore\libs\AceEvent-3.0\AceEvent-3.0.lua:119