fix for Process:isSuccessful()
merged branch tgabi333/2.2 (PR #8815)

This PR was merged into the 2.2 branch.

Discussion
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[Process] fix for Process:isSuccessful()

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

This is a rebase of https://github.com/symfony/symfony/pull/8801

If you call isSuccessful() on a running process you would get true because of 0 == null. I think that is not the expected behavior and that is not what phpdoc @return block said so.

Commits
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262879d fix for Process:isSuccessful()
Merge branch '2.2' into 2.3

* 2.2:
  [Locale] fixed build-data exit code in case of an error
  fixed request format of sub-requests when explicitely set by the developer (closes #8787)
  Sets _format attribute only if it wasn't set previously by the user.
  Exclude little words of 'ee' to 'oo' plural transformation
  fixed the format of the request used to render an exception
  Fix typo in the check_path validator
  added a missing use statement (closes #8808)
  fix for Process:isSuccessful()

Conflicts:
	UPGRADE-3.0.md
	src/Symfony/Component/Locale/Resources/data/build-data.php