#2595 Added null value handling (#2605)

#2596 2595 Added null value handling and use the same behaviour as for
"null"

Author: MrHinsh

src/MigrationTools.Clients.TfsObjectModel/Tools/FieldMappingTool/FieldMaps/FieldValueMap.cs

@@ -38,6 +38,11 @@ internal override void InternalExecute(WorkItemData source, WorkItemData target)
                     targetWi.Fields[Config.targetField].Value = Convert.ChangeType(Config.valueMapping["null"], t);
                     Log.LogDebug("FieldValueMap: [UPDATE] field value mapped {SourceId}:{SourceField} to {TargetId}:{TargetField}", source.Id, Config.sourceField, target.Id, Config.targetField);
                 }
+                else if (sourceVal != null && sourceVal.Value == null)
+                {
+                    targetWi.Fields[Config.targetField].Value = Convert.ChangeType(Config.valueMapping["null"], t);
+                    Log.LogDebug("FieldValueMap: [UPDATE] field value mapped {SourceId}:{SourceField} to {TargetId}:{TargetField}", source.Id, Config.sourceField, target.Id, Config.targetField);
+                }
                 else if (sourceVal != null && Config.valueMapping.ContainsKey(sourceVal.Value.ToString()))
                 {
                     targetWi.Fields[Config.targetField].Value = Convert.ChangeType(Config.valueMapping[sourceVal.Value.ToString()], t);