You can recover from 0 stars if you have a large fleet in transit. This isn’t an issue in scan-only-trading games. If you are KO then you can’t access the trade system - just tested this.
If you can trade with defeated AI empires then it’s a game bug (you shouldn’t be allowed to), if you cannot trade then it’s a UI bug. It’s a bit of a hassle to test since you need to create a new game and kill an AI.