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.

Commencez par les points d’entrée scrollTo... de cdk-virtual-scroll-viewport et examinez comment sont représentés l’initialisation et un viewport vide. Confirmez avec les maintainers si le comportement final doit être une erreur ou un avertissement dans la console, puis ajoutez une couverture ciblée montrant que le défilement avant l’initialisation n’échoue plus silencieusement.

Rédigé par le modèle d'indexation à partir du texte de l'issue.