fix(core): allow empty static directories

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
  • I really want to add tests but I don't know how to persist it, so see test plan below
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

It turns out that simply ensureDir is not sufficient to make build succeed with an empty static folder. To reproduce, do mkdir website/_dogfooding/emptyStaticDir, and add _dogfooding/emptyStaticDir to the staticDirectories list, then run yarn build -l en. Server build errors with:

unable to locate '/.../docusaurus/website/_dogfooding/emptyStaticDir/**/*' glob

This seems to be a bad thing to happen for a glob pattern, but just in case, I've simply filtered out all non-existent directories instead of creating empty ones. This also means the user doesn't magically have a static folder created after every build.

Test Plan

Tested locally with the above reproduction. I don't really know how to add a dogfooding test—my suspicion is it's impossible to do it in a simple way, because empty dirs aren't checked in to git... I just added a directory that doesn't exist, which should do half the job.

Related issues/PRs

#6311

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	65
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

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✅ [V2]

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[github-actions]

Size Change: +75 B (0%)

Total Size: 805 kB

Filename	Size	Change
website/build/assets/js/main.********.js	609 kB	+75 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	50.5 kB
website/build/assets/css/styles.********.css	107 kB
website/build/index.html	38.8 kB

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