Good question. The English rules state:
1. For the most unused coins:
The player who has the most unused coins in front of him gets one diamond. In the case of tie, all involved players receive one diamond each.
My call would be that no-one gets the diamond, for two possible reasons:
- If the diamond is compensation for the most wasted money in the buying phase, then no-one deserves it. Everyone spent all their money, and got value from it.
- If the rules mentioned the abstract 'money' then all players with zero might be tied for most. But the rules talk about physical coins, and that paragraph seems to exclude any player without any.