refactor(UserRep): nullable related propoerties gemacht.

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

DigitalData.UserManager.Application/DTOs/UserRep/UserRepCreateDto.cs

@@ -6,6 +6,6 @@ namespace DigitalData.UserManager.Application.DTOs.UserRep
         int? UserId,
         int? RepGroupId,
         int? GroupId,
-        int RepUserId
+        int? RepUserId
     ) : BaseCreateDto();
 }

DigitalData.UserManager.Application/DTOs/UserRep/UserRepReadDto.cs

@@ -11,7 +11,7 @@ namespace DigitalData.UserManager.Application.DTOs.UserRep
         int? RepUserId,
         int? RepGroupId,        
         string AddedWho,
-        string ChangedWho,        
+        string? ChangedWho,        
         UserReadDto? User,
         GroupReadDto? RepGroup,
         GroupReadDto? Group,

DigitalData.UserManager.Application/DTOs/UserRep/UserRepUpdateDto.cs

@@ -6,6 +6,6 @@ namespace DigitalData.UserManager.Application.DTOs.UserRep
         int? UserId,
         int? RepGroupId,
         int? GroupId,
-        int RepUserId
+        int? RepUserId
     ) : BaseUpdateDto();
 }