Fix reference empty guid

GerbilManagerWebAPI/Dtos/CreateLitterDto.cs

@@ -5,7 +5,7 @@ namespace GerbilManagerWebAPI.Dtos
         public required string Name { get; init; }
         public required DateTime Date { get; init; }
         public int? Strength { get; init; }
-        public Guid? Father { get; init; }
-        public Guid? Mother { get; init; }
+        public Guid Father { get; init; }
+        public Guid Mother { get; init; }
     }
 }