Bug 16337 – Posix get empty environment value returns null

If one saves an environment variable that is a blank string, and then retrieves that value, the returned value is the null string. While this normally would be fine (null is a valid blank string), the get function uses null as a sentinel to signify that the environment variable isn't present. This means one cannot distinguish between getting an environment variable that is blank, and one that is missing. PR: https://github.com/dlang/phobos/pull/4627