Fixes Issue #13184 - incremental output getters now return empty strings

fixed Issue #13184

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | Issue #13184 
| License       | MIT
| Doc PR        | [`Symfony\Component\Process\Process::getIncrementalOutput()`](http://api.symfony.com/2.6/Symfony/Component/Process/Process.html#method_getIncrementalOutput), [`Symfony\Component\Process\Process::getIncrementalErrorOutput()`](http://api.symfony.com/2.6/Symfony/Component/Process/Process.html#method_getIncrementalErrorOutput)