fix(plugin-vue): fix sourcemap when no script block in sfc (close #8601)

[meteorlxy]

Description

Fixes #8601

Before

When either scriptMap or templateMap is empty, it will cause the entire sourcemap to be empty.

After

Allow either scriptMap or templateMap to be empty.

Additional context

Also remove some redundant type annotations.

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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.
• 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.

[sapphi-red]

The implementation looks good to me. Could you add a test to playground/vue-sourcemap?

[meteorlxy]

Didn't notice we have tests for that. I'll add tests later.

[meteorlxy]

@sapphi-red done

[sapphi-red]

Thanks!