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"

0 files changed, 0 insertions, 0 deletions