[Chat] - Ensure it's possible to return an empty box for the bottom sheet in the scaffold

[matthew-carroll]

There may be times when a chat scaffold wants to momentarily hide its bottom sheet, such as when a user interacts with a text field in the content area. Make sure that the chat scaffolds support completely removing the bottom sheet, when desired.

This requirement was found in a client app where the client added AI responses in the content area that include text input.