fix(runtime-core): fix empty fragment in runtime-core

[Doctor-wu]

fix empty fragment in runtime-core
fix #10007

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	89.4 kB (+4 B)	34 kB (+3 B)	30.7 kB (+8 B)
vue.global.prod.js	146 kB (+4 B)	53.3 kB (+2 B)	47.6 kB (+2 B)

Usages

Name	Size	Gzip	Brotli
createApp	49.7 kB (+4 B)	19.5 kB (+2 B)	17.8 kB (+1 B)
createSSRApp	53.1 kB (+4 B)	20.8 kB (+3 B)	19 kB (-9 B)
defineCustomElement	52 kB (+4 B)	20.2 kB (+2 B)	18.5 kB (-46 B)
overall	63.2 kB (+4 B)	24.4 kB (+3 B)	22.2 kB (+3 B)