[BugFix][Spark] Fix the bug that VertexWrite does not generate vertex count file

[acezen]

Proposed changes

as title.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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Checklist

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Further comments

Fixed #109

[github-actions]

🎊 PR Preview 9f8079b has been successfully built and deployed to https://alibaba-graphar-build-pr-110.surge.sh

_{🤖 By surge-preview}

[lixueclaire]

LGTM, and please fix the minor issue (in the comments) before merging