fix(html): relative paths without leading dot wasn't rewritten

[sapphi-red]

Description

When the base is without trailing slash, relative path without leading dot wasn't working.

vite/playground/assets/index.html

Line 364 in 31333bb

<script src="asset/main.js" type="module"></script>

The first commit simply changes the base (this will make the test fail even with the paths in the test is updated).
The second commit is the actual fix.

This bug was found in #5657.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
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• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
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