Commented Unassigned: Allow to pass an empty ModelState into CreateErrorResponse [1091]

Would be nice if ```CreateErrorResponse(HttpStatusCode.BadRequest, ModelState)``` didn't throw an exception in case ModelState is empty.
Comments: ``` { "message":"Anerrorhasoccurred.", "exceptionMessage":"Themodelstateisvalid. Parametername:modelState", "exceptionType":"System.ArgumentException", "stackTrace":"atSystem.Web.Http.HttpError..ctor(ModelStateDictionarymodelState,BooleanincludeErrorDetail) atSystem.Net.Http.HttpRequestMessageExtensions.<>c__DisplayClassb.<CreateErrorResponse>b__a(BooleanincludeErrorDetail) atSystem.Net.Http.HttpRequestMessageExtensions.CreateErrorResponse(HttpRequestMessagerequest,HttpStatusCodestatusCode,Func`2errorCreator) atSystem.Net.Http.HttpRequestMessageExtensions.CreateErrorResponse(HttpRequestMessagerequest,HttpStatusCodestatusCode,ModelStateDictionarymodelState) atApp.Web.Controllers.RegisterController.Post(NewUseruser)inc:\Projects\SiteSDK\Source\Web\Controllers\RegisterController.cs:line46 atlambda_method(Closure,Object,Object[]) atSystem.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass13.<GetExecutor>b__c(Objectinstance,Object[]methodParameters) atSystem.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.Execute(Objectinstance,Object[]arguments) atSystem.Web.Http.Controllers.ReflectedHttpActionDescriptor.<>c__DisplayClass5.<ExecuteAsync>b__4() atSystem.Threading.Tasks.TaskHelpers.RunSynchronously[TResult](Func`1func,CancellationTokencancellationToken)" } ```