Skip closedDate field validation when closedDate field doesn't exist

Handle case where there is no closed date field in target workitem. For us it didn't work for Shared paramter. We cannot modify shared parameter workitem in azure devops. Skip field didn't work.

Author: Konop1992

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

@@ -895,6 +895,12 @@ private void CheckClosedDateIsValid(WorkItemData sourceWorkItem, WorkItemData ta
             {
                 closedDateField = "Microsoft.VSTS.Common.ClosedDate";
             }
+            else if (!targetWorkItem.ToWorkItem().Fields.Contains("System.ClosedDate"))
+            {
+                Log.LogDebug("CheckClosedDateIsValid::ClosedDate field doesn't exist in targetWorkItem: {targetWorkItem} - nothing to validate.", targetWorkItem);
+                return;
+            }
+
             Log.LogDebug("CheckClosedDateIsValid::ClosedDate field is {closedDateField}", closedDateField);
             if (targetWorkItem.ToWorkItem().Fields[closedDateField].Value == null && (targetWorkItem.ToWorkItem().Fields["System.State"].Value.ToString() == "Closed" || targetWorkItem.ToWorkItem().Fields["System.State"].Value.ToString() == "Done"))
             {
@@ -921,7 +927,6 @@ private void CheckClosedDateIsValid(WorkItemData sourceWorkItem, WorkItemData ta
                     Log.LogWarning("Target ClosedDate Field: ", "System.ClosedDate");
                 }
             }
-
         }
 
         private bool SkipRevisionWithInvalidIterationPath(WorkItemData targetWorkItemData)