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

Elwig/Models/Entities/DeliveryPartModifier.cs

@@ -14,12 +14,12 @@ namespace Elwig.Models.Entities {
         public int DPNr { get; set; }
 
         [ForeignKey("Year, DId, DPNr")]
-        public virtual DeliveryPart Part { get; private set; }
+        public virtual DeliveryPart Part { get; private set; } = null!;
 
         [Column("modid")]
-        public string ModId { get; set; }
+        public required string ModId { get; set; }
 
         [ForeignKey("Year, ModId")]
-        public virtual Modifier Modifier { get; private set; }
+        public virtual Modifier Modifier { get; private set; } = null!;
     }
 }