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"
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