Backend: Runner error when coverage_report is set to 'null'

Summary

Defining artifacts:reports:coverage_report as null is considered a valid configuration but crashes during pipeline execution.

Steps to reproduce

crashing-job:
  stage: test
  script:
    - []
  artifacts:
    reports:
      coverage_report: null

Example Project

What is the current bug behavior?

The pipeline runs, but the job fails with "The scheduler failed to assign job to the runner, please try again or contact system administrator"

What is the expected correct behavior?

Job runs and collects no artifacts. That is, the same behaviour as if artifacts:reports:coverage_report was never explicitly configured.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

GitLab Enterprise Edition 15.4.2-ee

Results of GitLab application Check

Implementation

backend	MR
Fix error when CI entry artifacts:reports:coverage_report is set to 'null'	!108478 (merged)