creates a user 0 for unknown to keep ref integrity

src/Umbraco.Core/Constants-Security.cs

@@ -12,6 +12,14 @@ namespace Umbraco.Core
             /// </summary>
             public const int SuperId = -1;
 
+            /// <summary>
+            /// The id for the 'unknown' user
+            /// </summary>
+            /// <remarks>
+            /// This is a user row that exists in the DB only for referential integrity but the user is never returned from any of the services
+            /// </remarks>
+            public const int UnknownId = 0;
+
             public const string AdminGroupAlias = "admin";
             public const string SensitiveDataGroupAlias = "sensitiveData";
             public const string TranslatorGroupAlias = "translator";