`DataDictionary.load()` throws exception on message with no fields

[chrjohn]

Describe the bug
The XMLnonFIX message (msgType n) has no body fields for most FIX versions since it uses header field XmlData to send data.

To Reproduce
Add the message type to a dictionary (it is not present in the QFJ dictionaries (for whatever reason)) and load it.

Caused by: quickfix.ConfigError: No fields found: msgType=n
	at quickfix.DataDictionary.load(DataDictionary.java:1174)
	at quickfix.DataDictionary.load(DataDictionary.java:1150)
	at quickfix.DataDictionary.read(DataDictionary.java:964)

Expected behavior
DataDictionary should not throw an exception.