fixed update function

api/Domain/Person.cs

@@ -31,10 +31,22 @@ public sealed class Person
         return new Person(username, realName, birthday);
     }
 
+    public void Update(string realName, DateOnly birthday)
+    {
+        realName = realName.Trim();
+
+        if (realName.Length == 0) throw new ArgumentException("Name cannot be empty");
+        if (!IsAtLeast16(birthday) || birthday >= DateOnly.FromDateTime(DateTime.Today))
+            throw new ArgumentException("Person must be at least 16 years old.");
+
+        RealName = realName;
+        Birthday = birthday;
+    }
+
     // Konstruktor zum Erzeugen von Nutzerobjekten aus der Datenbank
     internal static Person FromStorage(string username, string realName, DateOnly birthday)
         => new(username, realName, birthday);
-
+    
     public static string NormalizeUsername(string username)
         => username.Trim().ToLowerInvariant();
 

api/Services/PersonService.cs

@@ -45,11 +45,12 @@ namespace api.Services
 
         public async Task UpdatePersonAsync(string username, PersonDto personDto)
         {
+            Person person = await PersonExists(username);
 
-            await PersonExists(username);
+            string updatedName = string.IsNullOrWhiteSpace(personDto.RealName) ? person.RealName : personDto.RealName;
 
-            Person updatedPerson = Person.Create(username: username, realName: personDto.RealName, birthday: personDto.Birthday);
-            await _personRepository.UpdateAsync(updatedPerson);
+            person.Update(realName: updatedName, birthday: personDto.Birthday);
+            await _personRepository.UpdateAsync(person);
         }
 
         public async Task DeletePersonAsync(string username)