Welcome to the forum. The history can be empty the first time your app invokes startChat, and most cookbook examples do exactly that, or don’t even bother to pass history which then defaults to an empty list.
There is a scenario where history is non-empty when starting a chat session: when (today) you are continuing a chat session from, say, last week. That involves retrieving the history from a persistent object store and passing the resurrected history as a parameter to startChat to continue from where it was last week.
Hope that helps.