cask: return nil instead of "" if no caveats
cask: return nil instead of "" if no caveats

EricFromCanada/brew,claui/brew,vitorgalvao/brew,JCount/brew,konqui/brew,JCount/brew,vitorgalvao/brew,claui/brew,reitermarkus/brew,Homebrew/brew,claui/brew,mahori/brew,MikeMcQuaid/brew,sjackman/homebrew,konqui/brew,reitermarkus/brew,nandub/brew,sjackman/homebrew,vitorgalvao/brew,Homebrew/brew,konqui/brew,konqui/brew,sjackman/homebrew,sjackman/homebrew,MikeMcQuaid/brew,EricFromCanada/brew,vitorgalvao/brew,nandub/brew,nandub/brew,MikeMcQuaid/brew,Homebrew/brew,mahori/brew,EricFromCanada/brew,nandub/brew,JCount/brew,Homebrew/brew,mahori/brew,EricFromCanada/brew,JCount/brew,MikeMcQuaid/brew,mahori/brew,reitermarkus/brew,claui/brew,reitermarkus/brew