Fixed issue with Mono. Mono throws exception when null is assigned to Pr...

Fixed issue with Mono. Mono throws exception when null is assigned to Prefix.

Argument cannot be null.
Parameter name: key
XmlTransformer.XmlNodeException: Argument cannot be null.
Parameter name: key ---> System.ArgumentNullException: Argument cannot be null.
Parameter name: key
  at System.Xml.NameTable.Add (System.String key) [0x00000] in <filename unknown>:0
  at System.Xml.XmlAttribute.set_Prefix (System.String value) [0x00000] in <filename unknown>:0
  at XmlTransformer.XmlElementContext.ScrubTransformAttributesAndNamespaces (System.Xml.XmlNode node) [0x00000] in <filename unknown>:0
  at XmlTransformer.XmlElementContext.CreateCloneInTargetDocument (System.Xml.XmlNode sourceNode) [0x00000] in <filename unknown>:0
  at XmlTransformer.XmlElementContext.get_TransformNode () [0x00000] in <filename unknown>:0
  at XmlTransformer.Transform.get_TransformNode () [0x00000] in <filename unknown>:0
  at XmlTransformer.Replace.Apply () [0x00000] in <filename unknown>:0
  at XmlTransformer.Transform.ApplyOnce () [0x00000] in <filename unknown>:0
  at XmlTransformer.Transform.Execute (XmlTransformer.XmlElementContext context, System.String argumentString) [0x00000] in <filename unknown>:0
  --- End of inner exception stack trace ---
  Error during Replace
  Executing Replace (transform line 10, 40)