No longer answering with null, but 0

ref #193

src/models/responses/ResponseDonation.ts

@@ -55,7 +55,7 @@ export class ResponseDonation implements IResponse {
         this.id = donation.id;
         this.donor = donation.donor.toResponse();
         this.amount = donation.amount;
-        this.payedAmount = donation.payedAmount;
+        this.payedAmount = donation.payedAmount || 0;
         if (this.payedAmount < this.amount) {
             this.status = DonationStatus.OPEN;
         }