[Process] Use correct test for empty string in UnixPipes

Process sometimes omit `0` as the test use PHP's equality to check read string. This PR correctly check for empty string.

Fix for `master` branch also available in this repository.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -