refactor(UserRep): nullable related propoerties gemacht.

- RepUserId von UserRepCreateDto löschbar gemacht.
 - ChangedWho von UserRepReadDto löschbar gemacht.
 - RepUserId von UserRepUpdateDto löschbar gemacht.

DigitalData.UserManager.Domain/Entities/BaseEntity.cs

@@ -20,6 +20,7 @@ namespace DigitalData.UserManager.Domain.Entities
         [Column("CHANGED_WHO")]
         public string? ChangedWho { get; set; }
 
+        //TODO: assign it to default value in create dto, not here!
         [Column("ADDED_WHEN", TypeName = "datetime")]
         [DefaultValue("GETDATE()")]
         public DateTime AddedWhen { get; set; } = DateTime.Now;

DigitalData.UserManager.Domain/Entities/UserRep.cs

@@ -6,7 +6,6 @@ namespace DigitalData.UserManager.Domain.Entities
     [Table("TBDD_USER_REPRESENTATION", Schema = "dbo")]
     public class UserRep : BaseEntity
     {
-        [Required]
         [Column("USER_ID")]
         public int? UserId { get; set; }