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 https://github.com/facebook/docusaurus/issues/10212

Fix regression introduced with refactor in https://github.com/facebook/docusaurus/pull/9859

Test Plan

local test 😅

[netlify[bot]]

✅ [V2]

Name	Link
🔨 Latest commit	7de274e432044392cbbf34721f1602e1986819c7
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/667060533622910007cc3f9a
😎 Deploy Preview	https://deploy-preview-10225--docusaurus-2.netlify.app
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[github-actions[bot]]

⚡️ 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[bot]]

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 |

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