fix(theme): avoid rendering empty search container if site has no search plugin

[slorber]

Motivation

If a site has no search plugin, we shouldn't display the navbar search container container.

It adds useless paddings and shifts the other navbar items.

Not an ideal solution but at least it works. CSS :has is going to be cross-browser supported very soon.

fix #7990
fix #8889

Test Plan

local only

[netlify]

✅ [V2]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 86	🟢 97	🟢 92	🟢 100	🔴 33	Report
/docs/installation	🟠 80	🟢 98	🟢 92	🟢 100	🟠 89	Report

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	2b9031e
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/6522eb9de4869f0008f65aaa
😎 Deploy Preview	https://deploy-preview-9385--docusaurus-2.netlify.app
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[github-actions]

Size Change: +114 B (0%)

Total Size: 1.13 MB

Filename	Size	Change
website/build/assets/css/styles.********.css	113 kB	+66 B (0%)
website/build/assets/js/main.********.js	861 kB	+36 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	117 kB	0 B
website/build/index.html	41.4 kB	+12 B (0%)

_{compressed-size-action}

[slorber]

Replaced by simpler CSS solution :empty (#10061)