Fix invalidcastexception in multivalueconditionalmap (#2725)

Fix for typecast error if work item field type is not a string (e.g.,
integer) in fieldmap multivalueconditionalmap -> "Field map fault
System.InvalidCastException: Unable to cast object of type
'System.Int32' to type 'System.String'."

Author: MrHinsh

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

@@ -75,12 +75,12 @@ private bool fieldsValueMatch(Dictionary<string, string> fieldAndValues, WorkIte
             bool matches = true;
             foreach (string field in fieldAndValues.Keys)
             {
-                if ((string)workitem.Fields[field].Value != fieldAndValues[field])
+                if ((workitem.Fields[field].Value?.ToString() ?? string.Empty) != fieldAndValues[field])
                 {
                     matches = false;
                 }
             }
             return matches;
         }
     }
-}
+}