release-notes: only show blog link when previous_tag passed
release-notes: only show blog link when previous_tag passed

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