Fix superenv ENV[] hack
Fix superenv ENV[] hack

We override ENV[] to always return strings under superenv, because
legacy formulae assume that CFLAGS, etc. are non-nil.

However, the current implementation has a bug. If I simply concatenate
ENV['CFLAGS'] with another string, it mutates ENV['CFLAGS']:

  irb> ENV['CFLAGS']
  => ""
  irb> ENV['CFLAGS'] + 'a'
  => "a"
  irb> ENV['CFLAGS']
  => "a"

Instead, let's simply return an empty string if the key doesn't exist.
This is sufficient because the following are equivalent:

  1. ENV['CFLAGS'] += "string"
  2. ENV['CFLAGS'] = ENV['CFLAGS'] + "string"

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