feat(cdk/scrolling): Scroll functions silently fail if the viewport is not yet fully initialized

#### Feature Description

`cdk-virtual-scroll-viewport`'s `scrollTo...` functions (like `scrollToIndex`) silently fail when the viewport is not yet initialized. Technically the functions are doing nothing wrong from a code standpoint, but since the component is not yet fully initialized, it is empty, so trying to scroll will do nothing meaningful. Sending back an error (or at least logging a warning to the console) if an empty viewport is scrolled would be helpful so that developers know that something went wrong.

This would be mitigated if we had an async option that managed the timing of scroll commands. See [this feature request](https://github.com/angular/components/issues/23715).

#### Use Case

This would improve every use case of the virtual scroller.

从 cdk-virtual-scroll-viewport 的 scrollTo... 入口点开始，检查初始化和空 viewport 是如何表示的。与维护者确认完成后的行为应该是错误还是控制台警告，然后添加有针对性的覆盖，表明初始化前滚动不再静默失败。

由索引模型根据 Issue 内容生成。