fix(content-docs): restore behavior when pagination front matter is null

[Josh-Cena]

Motivation

Fix #6103

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Added one fixture test

[netlify]

✔️ [V2]

🔨 Explore the source changes: a5081e5

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	74
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-6124--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 652 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	37.8 kB
website/build/assets/css/styles.********.css	101 kB
website/build/assets/js/main.********.js	484 kB
website/build/index.html	29.3 kB

_{compressed-size-action}

[netlify]

✔️ [V2]

🔨 Explore the source changes: 165ac7f

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/61bd433576e7e40007443599

😎 Browse the preview: https://deploy-preview-6124--docusaurus-2.netlify.app