Description:
------------
Empty environment variables passed to proc_open are filtered out.

Test script:
---------------
<?php
// process.php

$env = ['FOO' => 'abc', 'BAR' => ''];
$pipes = [];
$process = proc_open('./env.php', [], $pipes, `pwd`, $env);
proc_close($process);


#!/usr/bin/php
<?php
// env.php

var_dump($_SERVER['FOO']);
var_dump($_SERVER['BAR']);
php process.php will output:

string(3) "abc"
PHP Notice:  Undefined index: BAR in /Users/kuba/Projects/symfony/issues/env.php on line 6
NULL