Ensure ansible_job_id exists even for not-started jobs

This will fix gh-7432. 

Git bisect indicates that this was originally broken [here](https://github.com/ansible/ansible/commit/e09313120c96c00980aef12c3105647bdc3857ca), where previously the job id was always set to None initially. I am unsure of the semantics of  `self.runner.vars_cache`, so I did the simpler (but perhaps less robustly correct) thing.