Do not create pipelines with only .pre/.post jobs

Summary

Pipelines that have jobs only from the .pre/.post stages should be considered empty and not persisted.

Steps to reproduce

pre-checks:
  stage: .pre
  script:
    - echo "running a job from .pre stage"

post-checks:
  stage: .post
  script:
    - echo "running a job from .post stage"

This config should not trigger any pipelines