No null check on `state` in vote functions can cause crash

[MrBMRBLX]

In fastchat/serve/gradio_web_server.py, the vote_last_response function calls state.dict() without checking if state is None first.

After clear_history sets state = None, if a user clicks a vote button quickly before the UI updates, this will crash with AttributeError.

def vote_last_response(state, vote_type, model_selector, request):
    data = {
        "state": state.dict(),  # crashes if state is None
    }