command: fail early if no command is given
command: fail early if no command is given

Closes Homebrew/homebrew#44450.

Signed-off-by: Baptiste Fontaine <bfee279af59f3e3f71f7ce1fa037ea7b90f93cbf@yahoo.fr>

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