shared: handle missing Xcode/CLT version.
shared: handle missing Xcode/CLT version.

Fixes https://github.com/Homebrew/homebrew-core/issues/3973.

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