fixed model state filter

Luticate2.Utils/Middlewares/LuModelStateFilter.cs

                 var errors = "";
                 foreach (var key in context.ModelState.Keys)
                 {
-                    var error = " " + key + ": " + context.ModelState[key]
-                        .Errors.Select(modelError => modelError.ErrorMessage)
-                        .Aggregate((s, s1) => s + ", " + s1);
-                    errors += error;
+                    var state = context.ModelState[key];
+                    if (state.Errors.Count > 0)
+                    {
+                        var error = " " + key + ": " + state.Errors.Select(modelError => modelError.ErrorMessage)
+                                        .Aggregate((s, s1) => s + ", " + s1);
+                        errors += error;
+                    }
                 }
                 throw new LuResultException(LuResult<object>.Error(LuStatus.InputError, "Validation error:" + errors, ""));
             }