bug symfony#25869 [Process] Skip environment variables with false value in Process (francoispluchino)

This PR was merged into the 3.3 branch.

Discussion
----------

[Process] Skip environment variables with false value in Process

| Q             | A
| ------------- | ---
| Branch?       | master, 4.0, 3.3, 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

With the commit 03adce2, all env variables are injecting in the process, and the env variable with the `false` value are converted in empty string.

For example, it's problematic with the bundle SymfonyWebServerBundle and the last version of the [symfony/framework-bundle recipe](https://github.com/symfony/recipes/blob/master/symfony/framework-bundle/3.3/public/index.php#L11), because the WebServer override the value of `APP_ENV` with `false` to override previously loaded variables (c5a1218), and with the commit 03adce2, the `APP_ENV` variable is injected with a empty string value, instead of not being injected, as was the case before.

This PR not use the same logic as the [getDefaultEnv()](https://github.com/symfony/symfony/blob/4.0/src/Symfony/Component/Process/Process.php#L1553) method.

Commits
-------

2daf4f9 [Process] Skip environment variables with false value in Process

Author: nicolas-grekas

src/Symfony/Component/Process/Process.php

@@ -331,7 +331,9 @@ public function start(callable $callback = null/*, array $env = array()*/)
         } else {
             $envPairs = array();
             foreach ($env as $k => $v) {
-                $envPairs[] = $k.'='.$v;
+                if (false !== $v) {
+                    $envPairs[] = $k.'='.$v;
+                }
             }
         }