fix(custom-element): prevent injecting child styles if shadowRoot is false

[edison1105]

close #12630

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	100 kB (+27 B)	38 kB (+6 B)	34.3 kB (+46 B)
vue.global.prod.js	158 kB (+27 B)	57.8 kB (+6 B)	51.4 kB (-3 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.4 kB	18.2 kB	16.6 kB
createApp	54.3 kB	21.2 kB	19.3 kB
createSSRApp	58.5 kB	22.9 kB	20.9 kB
defineCustomElement	59.2 kB (+27 B)	22.8 kB (+8 B)	20.7 kB (+3 B)
overall	68.4 kB	26.3 kB	24 kB

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commit: 3684d65