docs: fix nullable json schema

[samchungy]

Changes

• Enables nullable JSON schema to be generated as the appropriate type: [TYPE, 'null'].

I'm not sure if all or other schemas are nullable but I went with a workaround to avoid completely re-writing the entire codebase.

Context

I was trying to utilise the "$schema": "https://docs.renovatebot.com/renovate-schema.json" field but my IDE was flagging a nullable field as invalid.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

Ran the generate schema command locally and viewed the rendered schema:

[CLAassistant]

All committers have signed the CLA.

[rarkins]

Can you post a diff of the resulting json schema file?

[samchungy]

Can you post a diff of the resulting json schema file?

Good call, caught something I missed.

Here's the new diff: samchungy@69d13af

[viceice]

LGTM

[renovate-release]

🎉 This PR is included in version 38.122.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 38.122.0

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