cmd/leaves: handle missing formula.
cmd/leaves: handle missing formula.

Use `runtime_formula_dependencies` which does this for us.

Fixes #6827.

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