How can we check after desearilizing json that particular variable in object is nothing

string json = "[{\"BatchId\":5126,\"ProcessId\":332432,\"FileType\":\"Charge\",\"SourceName\":\"MOT\",\"SubClient\":\" MAP8\",\"ScanDt\":\"\\/Date(1412726400000)\\/\",\"MRN#\":\"Code_57891\",\"Last name\":null,\"First Name\":null,\"Location\":null,\"Department\":null,\"Status\":null,\"Visit num\":null,\"TM\":null,\"ATIME\":null,\"OAPTYP\":null,\"AVISFLG\":null,\"APTYP\":null,\"Scheduling_Prov\":null,\"Age\":null,\"Rendering Provider\":null,\"Billable Provider\":null,\"DOB\":null,\"DOS\":null,\"ACCT_CREATE_DATETIME\":null,\"LW account#\":null,\"Other Num\":null,\"ACCT_NUM\":null,\"Age in Days\":null}]";
var serializer = new Ja ...