fix(emptyOutDir): never remove .git

[aleclarson]

Description

If .git exists in the root of outDir, assume the user doesn't want it removed. The rationale is that Vite never generates .git for the user, so it should leave it alone.

This PR also contains a fix related to emptyOutDir when the --watch flag is used in build mode. Previously, the outDir would not be emptied before Rollup rebuilds if emptyOutDir was null. Now it is emptied.

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-dev]

See #3145 that also fixes this issue while replacing rollup watch with chokidar. We could still merge this fix though in the meantime.