[Process] Use correct test for empty string in UnixPipes
[Process] Fix unit tests on Windows platform
[Process] Adjust PR #11264, make it Windows compatible and fix CS
bug #11381 [2.3] [Process] Use correct test for empty string in UnixPipes (whs, romainneutron)

This PR was merged into the 2.3 branch.

Discussion
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[2.3] [Process] Use correct test for empty string in UnixPipes

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This PR supersedes #11264 : 2.3 compatibility + Windows compatibility + CS fix

Commits
-------

cec0a45 [Process] Adjust PR #11264, make it Windows compatible and fix CS
9e1ea4a [Process] Use correct test for empty string in UnixPipes
Merge branch '2.3' into 2.4

* 2.3:
  [Process] Adjust PR #11264, make it Windows compatible and fix CS
  [Process] Fix unit tests on Windows platform
  bumped Symfony version to 2.3.19
  updated VERSION for 2.3.18
  update CONTRIBUTORS for 2.3.18
  updated CHANGELOG for 2.3.18
  [Process] Use correct test for empty string in UnixPipes

Conflicts:
	src/Symfony/Component/HttpKernel/Kernel.php
	src/Symfony/Component/Process/Tests/AbstractProcessTest.php