utils: fix which_editor.
utils: fix which_editor.

Handle when there's no editor variable specified or it's specified but
empty.

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