src: remove empty name check in node_env_var.cc

The empty name check has been removed since this has landed:
https://github.com/libuv/libuv/pull/2473

PR-URL: https://github.com/nodejs/node/pull/36133


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>