fix(v2): Don't error if site has no dependencies list

[jknoxville]

We noticed that when a site has no dependencies in its package.json, you sometimes get errors when running yarn start.

Oddly, its intermittent, seems to occur the first or second time you run yarn start after a yarn install, but not consistently. We observed it on the flipper website which only has devDependencies, and no dependencies array.

I think it's reasonable that a lot of sites only need devDependencies so this prevents erroring in those cases.

Have you read the [Contributing Guidelines on pull requests]?

Yes

Test Plan

Happy case still succeeds:

cd docusaurus
yarn
cd website
yarn start

I'm unfortunately submitting this fix blind as the issue is intermittent and hard to reproduce.
This is a stack trace from when the problem occurred, which pretty clearly points to where it's happening:

/Users/mweststrate/fbsource/xplat/sonar/website/node_modules/@docusaurus/core/bin/docusaurus.js:50
  const siteDocusaurusPackagesForUpdate = Object.keys(sitePkg.dependencies)
                                                 ^

TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at Object.<anonymous> (/Users/mweststrate/fbsource/xplat/sonar/website/node_modules/@docusaurus/core/bin/docusaurus.js:50:50)

Searching the code for dependencies doesn't show any other references except for the migration script which already deals with the undefined case.

[netlify]

✔️ [V2]
Built without sensitive environment variables

🔨 Explore the source changes: f750e8e

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60f9464b1fb830000788d5c5

😎 Browse the preview: https://deploy-preview-5205--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	80
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5205--docusaurus-2.netlify.app/

[Josh-Cena]

Er, duplicate of #5204? I think spreading dependencies and also introducing devDependencies looks like a better fix.

[jknoxville]

Whoops sorry, closing this one.

[slorber]

I see thanks, it's the upgrade helper that runs from time to time.

Not sure why your site has no dependency, not even docusaurus packages though? 🤪