refactor: simplify the reassignment of map

[Aaron-zon]

simplify the reassignment of map

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	100 kB	38.1 kB	34.3 kB
vue.global.prod.js	158 kB	58 kB	51.7 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.5 kB	18.2 kB	16.7 kB
createApp	54.6 kB	21.3 kB	19.4 kB
createSSRApp	58.8 kB	23 kB	21 kB
defineCustomElement	59.4 kB	22.9 kB	20.8 kB
overall	68.6 kB	26.4 kB	24 kB

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commit: 0d96006

[Justineo]

Thank you for contributing to Vue. However, please avoid stylistic changes unless they noticeably improve readability.