Use default of weight=1 for burndown charts
The release post for burndown charts says "When using this feature, make sure your weights have been properly assigned, since an open issue with no weight would contribute zero to the cumulative value." How about we count unknown weights as 1? It's a more reasonable default, and allows kanban-esque burndown charts with zero effort. AFAIK, people don't use weights less than 1 on purpose.