bug(cdk-virtual-scroll): use ngIf inside cdk-virtual-scroll-viewport break cdkVirtualFor render datas

#### Reproduction

https://stackblitz.com/edit/angular-zsbplw-8een6b?file=src/app/cdk-virtual-scroll-context-example.html

Steps to reproduce:
1. click empty button
2. click rebuild button (datas no render)
3. refresh, click empty button and then click rebuild more datas button (datas render correct)

#### Expected Behavior

What behavior were you expecting to see?
datas should render correct

#### Actual Behavior

What behavior did you actually see?
no datas render

#### Environment

- Angular: 11.1.1
- CDK/Material: 11.1.1
- Browser(s): Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): Windows

从链接的 StackBlitz 复现开始，重点查看 src/app/cdk-virtual-scroll-context-example.html 以及 cdk-virtual-scroll-viewport/cdkVirtualFor 入口点。复现 empty-button 然后 rebuild 的序列，并将其与 refresh/more-data 序列进行比较。当 virtual-scroll 数据在两个序列中都能正确渲染时，即表示完成。

由索引模型根据 Issue 内容生成。