fix(v2): Allow null as valid for title of item in footer links.

[ben-qnimble]

Motivation

I want to make the category title in the footer links disappear. This is particularly useful when you do not have many links, so displaying category headings for the links isn't necessarily appropriate. Conveniently, the footer plugin already handles this by testing if the title is null and if so skipping writing the h4 section. Thus, the only change is to allow a null value for the title in the classic theme validation.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

I have tested this change change by setting a footer title to both to null:

 footer: {
      style: 'dark',
      copyright: `Copyright © ${new Date().getFullYear()}`,
      links: [
        {
          title: null,
          items: [
            {
              label: 'Some Page',
              to: 'page',
            },
          ],
        },
       ],
}

and verifying that there are no error and that the title is not displayed.

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[github-actions]

Size Change: 0 B

Total Size: 544 kB

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Filename	Size	Change
website/build/assets/css/styles.********.css	87 kB	0 B
website/build/assets/js/main.********.js	370 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	60.5 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	25.6 kB	0 B

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	91
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4267--docusaurus-2.netlify.app/classic/

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[slorber]

make sense, thanks