Allow @chat.transform_assistant_response function to return None

[wch]

This allows the @chat.transform_assistant_response function to return None, in which case nothing is sent to the client (and it will not update the output).

It also fixes a small issue where, when streaming, the last message would not be displayed. (This issue only manifests when the transform function returns None, because previously the last piece of text would get sent twice, once with chunk_type=None and once with chunk_type="message_end".

[cpsievert]

After chatting with @wch, we discovered/remembered this is needed for @wch's hand-rolled throttling implementation, but it's also useful if want to transform the content when the message is done, for example:

from shiny.express import ui

chat = ui.Chat(id="chat")
chat.ui()


@chat.transform_assistant_response
def transform(content: str, chunk: str, done: bool) -> str | None:
    if done:
        return content + "...DONE!"
    else:
        return content


@chat.on_user_submit
async def _():
    await chat.append_message_stream(("Basic ", "response"))