bump-formula-pr: fix regression when formula.tap is Nil
bump-formula-pr: fix regression when formula.tap is Nil

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