[Process] Fixed fatal errors in getOutput and getErrorOutput when process was not started 

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10022
| License       | MIT

This PR replaces #9452 and address the latest changes.
Side note : I've not updated `getExitCode`, `getExitCodeText` and `isSuccessful` as they were explicitly tested to return null in case the process is not started or terminated. I think it would be a BC break to do that.