fix(core): fix sites unable to start/build without a static dir

[slorber]

Motivation

A site should be able to start/build without a /static dir

Fix #10212

Fix regression introduced with refactor in #9859

Test Plan

local test 😅

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	7de274e
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/667060533622910007cc3f9a
😎 Deploy Preview	https://deploy-preview-10225--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 76	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 64	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 74	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 69	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 67	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 71	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/tags	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report

[github-actions]

Size Change: 0 B

Total Size: 1.84 MB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/codeTranslations.json	2 B
website/.docusaurus/docusaurus.config.mjs	27.1 kB
website/.docusaurus/globalData.json	123 kB
website/.docusaurus/i18n.json	930 B
website/.docusaurus/registry.js	304 kB
website/.docusaurus/routes.js	202 kB
website/.docusaurus/routesChunkNames.json	130 kB
website/.docusaurus/site-metadata.json	2.17 kB
website/build/assets/css/styles.********.css	112 kB
website/build/assets/js/main.********.js	897 kB
website/build/index.html	38.1 kB

_{compressed-size-action}