Allow CI `only`/`except` to be combined with `rules` when null

What does this MR do and why?

Per #215662 (closed), users would like to be able to override an extended job's only/except or rules keyword with null. Currently, the problem is that the CI validator is giving a validation error that prevents only/except and rules to be used together even if their values are nulled.

This MR implements the following:

• Updated the FE CI json schema so that you can set rules to null
• Updated the relevant validation statement so that it only applies when rules is not null
• Modified the disallowed_keys validator to ignore keys with null values

Note that a blank value or ~ is equivalent to the Yaml null. In our examples below, we are explicitly setting the value as null.

Screenshots or screen recordings

• Example config:

job-a:
  rules:
    - if: '$CI_MERGE_REQUEST_ID'
  script: echo "foo"

job-b:
  extends: job-a
  rules: null
  only:
    refs:
      - master
  except:
    refs:
      - merge_requests
  script: echo "bar"

• Old Behaviour
  • Shows error message jobs:job-b config key may not be used with `rules`: only, except
  • rules: null has an FE validation error

• New Behaviour
  • No validation errors

How to set up and validate locally

1. Test the following config examples in your Project's CI/CD editor.

• Config Example 1: rules combined with null only/except

.base-job:
  only:
    refs:
      - master
  except:
    variables:
      - $CI_JOB_NAME != "" # Always true, so .base-job should never run
  script: echo "base-job"
  
job1:
  extends: .base-job
  only: null
  except: null
  rules:
    - if: $CI_JOB_NAME == ""
  script: echo "job1"

job2:
  extends: .base-job
  only: null
  except: null
  rules:
    - if: $CI_JOB_NAME == "job2"
  script: echo "job2"

• Config Example 2: only/except combined with null rules

.base-job:
  rules:
    - if: $CI_JOB_NAME == "" # Always false, so .base-job should never run
  script: echo "base-job"
  
job1:
  extends: .base-job
  rules: null
  only:
    refs:
      - master
  except:
    variables:
      - $CI_JOB_NAME != ""
  script: echo "job1"

job2:
  extends: .base-job
  rules: null
  only:
    refs:
      - master
  except:
    variables:
      - $CI_JOB_NAME == ""
  script: echo "job2"

2. Observe that they do not produce validation errors.
3. Commit the change (to run the pipeline) and see that only job2 runs, which indicates the .base-job's only/except/rules values are nulled and the overidden values take effect.
4. Also observe that if you replace the null with a non-null value, then the validation error message displays as expected.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #215662 (closed)