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

Start with the linked StackBlitz reproduction, especially src/app/cdk-virtual-scroll-context-example.html and the cdk-virtual-scroll-viewport/cdkVirtualFor entry points. Reproduce the empty-button then rebuild sequence, and compare it with the refresh/more-data sequence. Done means virtual-scroll data renders correctly in both sequences.

Written by the indexing model from the issue text.