fix: allow `artifacts: null` in gitlab ci schema

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA

What does this MR do and why?

This MR adds "null" as a valid argument for the artifacts keyword in the editor schema for CI pipelines. This fixes #436462 (closed).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Download the changed ci.json file into your project root and add the following to your settings.json in VS Code:

{
    "yaml.schemas": {
        "./ci.json" : ["*.yml", "*.yaml"]
    },
}