[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
----------

[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
Merge branch '2.4' into 2.5

* 2.4:
  [Process] Adjust PR #11264, make it Windows compatible and fix CS
  [Process] Fix unit tests on Windows platform
  bumped Symfony version to 2.4.9
  bumped Symfony version to 2.3.19
  updated VERSION for 2.4.8
  updated CHANGELOG for 2.4.8
  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