src: fix empty-named env var assertion failure

Setting an environment variable with an empty name on Windows resulted
in an assertion failure, because it was checked for an '=' sign at the
beginning without verifying the length was greater than 0.

Fixes: https://github.com/nodejs/node/issues/32920
Refs: https://github.com/nodejs/node/pull/27310

PR-URL: https://github.com/nodejs/node/pull/32921


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>