Entities: Use 'required' and '= null!' to get rid of warnings

Elwig/Models/Entities/PaymentDeliveryPartBucket.cs

@@ -36,9 +36,9 @@ namespace Elwig.Models.Entities {
         }
 
         [ForeignKey("Year, AvNr")]
-        public virtual PaymentVar Variant { get; private set; }
+        public virtual PaymentVar Variant { get; private set; } = null!;
 
         [ForeignKey("Year, DId, DPNr")]
-        public virtual DeliveryPart DeliveryPart { get; private set; }
+        public virtual DeliveryPart DeliveryPart { get; private set; } = null!;
     }
 }