ISWebutilities.DeserialiseXmlStringToObject; better handling of Blank or Nothing XML string

If a blank String or Nothing (Null) is passed into ISWebutilities.DeserialiseXmlStringToObject as the xml parameter, it fails and the following error is returned "Failed to deserialise XML string to object. String reference not set to an instance of a String. Parameter name: s".

It is not obvious what the error is, so the message returned is now "Failed to deserialise XML string to object. XML String is blank".