fix(compiler-dom): avoid stringify `<option>` with null value

[edison1105]

close #12093

[github-actions[bot]]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	101 kB	38.1 kB	34.3 kB
vue.global.prod.js	160 kB	58 kB	51.5 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	49 kB	18.9 kB	17.2 kB
createApp	55.6 kB	21.4 kB	19.6 kB
createSSRApp	59.6 kB	23.1 kB	21 kB
defineCustomElement	60.4 kB	23 kB	20.9 kB
overall	69.3 kB	26.5 kB	24.1 kB

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@vue/compiler-core

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@vue/compiler-dom

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@vue/compiler-sfc

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@vue/reactivity

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@vue/runtime-core

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@vue/compiler-ssr

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@vue/runtime-dom

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@vue/server-renderer

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@vue/shared

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vue

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@vue/compat

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commit: 4eba15b