[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.