Do not default "retry" to 0 when no "max" specified

Proposal

Documentation: https://docs.gitlab.com/ci/yaml/?query=runner_system_failure#retry

When defining "retry" as per below the job will not be retried as the "retry" will default to 0. This seems counter-intuitive, a retry of 0 basically will not retry at all.

job:
...
  retry:
    when:
    - runner_system_failure

If job is defined as per below then the job is retried:

  retry:
    max: 2
    when:
      - runner_system_failure