UsersToMap can be null (#2545)

🐛 (TfsWorkItemMigrationProcessor.cs): add null check for usersToMap to
prevent potential null reference exception

The change adds a null check for the `usersToMap` object before
accessing its `IdentityMap` property. This prevents a potential null
reference exception if `usersToMap` is null, ensuring the application
handles such cases gracefully and improves overall stability.

FYI @satano 

Closes: #2544
Discussion:
#2540

src/MigrationTools.Clients.TfsObjectModel/Processors/TfsWorkItemMigrationProcessor.cs

@@ -229,7 +229,7 @@ private void ValidateAllUsersExistOrAreMapped(List<WorkItemData> sourceWorkItems
         {
             contextLog.Information("Validating::Check that all users in the source exist in the target or are mapped!");
             IdentityMapResult usersToMap = CommonTools.UserMapping.GetUsersInSourceMappedToTargetForWorkItems(this, sourceWorkItems);
-            if (usersToMap.IdentityMap != null && usersToMap.IdentityMap.Count > 0)
+            if (usersToMap != null && usersToMap.IdentityMap != null && usersToMap.IdentityMap.Count > 0)
             {
                 Log.LogWarning("Validating Failed! There are {usersToMap} users that exist in the source that do not exist "
                     + "in the target. This will not cause any errors, but may result in disconnected users that could have "