fix(useVirtualList): 修复当itemHeight为number时，list变化滚动条位置

[dan1ao]

修复当itemHeight为number时，list变化后offset计算错误。

[中文版模板 / Chinese template]

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Language	Changelog
🇺🇸 English	Fix the offset calculation error after the list changes when itemHeight is number
🇨🇳 Chinese	修复当itemHeight为number时，list变化后offset计算错误

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[dan1ao]

复现步骤：初始list.length为100，容器内可以显示10个，滚动到第N「N>10」个，list.length变化为<N，targetList为[],实际显示空白，wrapperTarget内为空。