Fix one of DCS constructors that cause NRE in wcf

Author: khdang

src/System.Private.DataContractSerialization/src/System/Runtime/Serialization/DataContractSerializer.cs

@@ -50,7 +50,8 @@ public DataContractSerializer(Type type, string rootName, string rootNamespace)
 
         public DataContractSerializer(Type type, string rootName, string rootNamespace, IEnumerable<Type> knownTypes)
         {
-            Initialize(type, new XmlDictionaryString(null, rootName, 0), new XmlDictionaryString(null, rootNamespace, 0), knownTypes, int.MaxValue, false, false, null, false);
+            XmlDictionary dictionary = new XmlDictionary(2);
+            Initialize(type, dictionary.Add(rootName), dictionary.Add(DataContract.GetNamespace(rootNamespace)), knownTypes, int.MaxValue, false, false, null, false);
         }
 
 

src/System.Private.DataContractSerialization/src/System/Xml/XmlDictionaryString.cs

@@ -26,7 +26,7 @@ public class XmlDictionaryString
         public XmlDictionaryString(IXmlDictionary dictionary, string value, int key)
         {
             if (dictionary == null)
-                _dictionary = new EmptyStringDictionary();
+                throw System.Runtime.Serialization.DiagnosticUtility.ExceptionUtility.ThrowHelperError(new ArgumentNullException("dictionary"));
             if (value == null)
                 throw System.Runtime.Serialization.DiagnosticUtility.ExceptionUtility.ThrowHelperError(new ArgumentNullException("value"));
             if (key < MinKey || key > MaxKey)