process: fix reading zero-length env vars on win32

Up until now, Node did not clear the current error code
attempting to read environment variables on Windows.
Since checking the error code is the way we distinguish between
missing and zero-length environment variables, this could lead to a
false positive when the error code was still tainted.

In the simplest case, accessing a missing variable and then a
zero-length one would lead Node to believe that both calls yielded
an error.

Before:

    > process.env.I=''; process.env.Q; process.env.I
    undefined
    > process.env.I=''; /*process.env.Q;*/ process.env.I
    ''

After:

    > process.env.I=''; process.env.Q; process.env.I
    ''
    > process.env.I=''; /*process.env.Q;*/ process.env.I
    ''

This only affects Node 8 and above, since before
1aa595e5 we always constructed a
`v8::String::Value` instance for passing the lookup key to the OS,
which in in turn always made a heap allocation and therefore
reset the error code.

PR-URL: https://github.com/nodejs/node/pull/18463


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>