All impure environment variables are initialized as part of `initEnv()` to an empty value if they are unset. Whenever `NIX_SSL_CERT_FILE` is set, this cause a spurious warning. Let's document it here as a test before we fix it. Change-Id: Iccba8ccb487d9cc25490e940ff09c1c5a13ae097 Signed-off-by: Raito Bezarius <raito@lix.systems>