[Process] Fix two process-builder minor bugs

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Everything is in the commit messages : 

 - Use new ProcessUtils::escapeArgument to escape ProcessBuillder prefix
 - Do not throw LogicException in ProcessBuilder::getProcess if no arguments are set but a prefix is