fix: cli upgrade helper fail when no package.dependencies

[mweststrate]

Motivation

In Flipper, https://github.com/facebook/flipper/blob/master/website/package.json, all our dependencies live in devDependecies of our package.json. As a result dependencies is not set, causing a crash when running yarn start:

/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)
    at Module._compile (internal/modules/cjs/loader.js:955:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:991:10)
    at Module.load (internal/modules/cjs/loader.js:811:32)
    at Function.Module._load (internal/modules/cjs/loader.js:723:14)
    at Function.Module.runMain (internal/modules/cjs/loader.js:1043:10)
    at internal/main/run_main_module.js:17:11
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

Setting "dependencies": {}, works as work around, but this patch fixes the crash, and makes sure the deps are upgraded as well if they live in devDependencies instead of dependencies.

Have you read the Contributing Guidelines on pull requests?

skimmed

Test Plan

yarn install && yarn start in the flipper website/ dir, per link above.

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[netlify]

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

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

Lighthouse ran on https://deploy-preview-5204--docusaurus-2.netlify.app/

[slorber]

LGTM thanks 👍

Was wondering why you put docusaurus packages as devDependencies? It's unusual

[mweststrate]

Just checked, doesn't seem to be a specific rationale behind it, but just there as devDependency since original generation early 2018 with docusaurus 1.0.9. Semantically speaking I'd say it is quite correct to have them as devDependencies, since these are all(?) build-only deps. But that gets into the nit picking territory 😅.

....Unless dependabot would finally stop running false security errors havoc on devDependencies, then it would suddenly add a lot of value to use those instead of normal dependencies 😆

[slorber]

I see thanks :)

I guess it's a matter of perspective. IMHO for an SSG we can consider the CI (Netlify/Vercel/GH actions) is the production env, and can leverage yarn install --production to install faster and skip pure dev things like eslint and prettier :)