Make `PATH#validate` return `nil` if empty.
Make `PATH#validate` return `nil` if empty.

claui/brew,amar-laksh/brew_sudo,jmsundar/brew,EricFromCanada/brew,gregory-nisbet/brew,reelsense/brew,JCount/brew,MikeMcQuaid/brew,reelsense/brew,maxim-belkin/brew,hanxue/linuxbrew,konqui/brew,alyssais/brew,palxex/brew,muellermartin/dist,MikeMcQuaid/brew,MikeMcQuaid/brew,Linuxbrew/brew,zmwangx/brew,DomT4/brew,zmwangx/brew,hanxue/linuxbrew,amar-laksh/brew_sudo,JCount/brew,vitorgalvao/brew,reitermarkus/brew,tonyg/homebrew,rwhogg/brew,nandub/brew,vitorgalvao/brew,claui/brew,claui/brew,EricFromCanada/brew,sjackman/homebrew,Homebrew/brew,Linuxbrew/brew,DomT4/brew,mgrimes/brew,vitorgalvao/brew,sjackman/homebrew,tonyg/homebrew,reelsense/brew,vitorgalvao/brew,reitermarkus/brew,mahori/brew,ilovezfs/brew,pseudocody/brew,amar-laksh/brew_sudo,ilovezfs/brew,gregory-nisbet/brew,zmwangx/brew,claui/brew,mgrimes/brew,jmsundar/brew,aw1621107/brew,maxim-belkin/brew,alyssais/brew,aw1621107/brew,DomT4/brew,JCount/brew,aw1621107/brew,jmsundar/brew,nandub/brew,muellermartin/dist,DomT4/brew,mgrimes/brew,Homebrew/brew,maxim-belkin/brew,alyssais/brew,bfontaine/brew,bfontaine/brew,hanxue/linuxbrew,palxex/brew,mahori/brew,pseudocody/brew,rwhogg/brew,nandub/brew,Linuxbrew/brew,mahori/brew,EricFromCanada/brew,mahori/brew,Homebrew/brew,nandub/brew,reitermarkus/brew,sjackman/homebrew,konqui/brew,sjackman/homebrew,MikeMcQuaid/brew,rwhogg/brew,reitermarkus/brew,ilovezfs/brew,gregory-nisbet/brew,Linuxbrew/brew,bfontaine/brew,palxex/brew,tonyg/homebrew,konqui/brew,muellermartin/dist,Homebrew/brew,EricFromCanada/brew,konqui/brew,JCount/brew,pseudocody/brew