Fix/entities are not optional in maltego transform request message

"Major" bug discovered, when parsing XML messages from Maltego.

See commit messages. 

Attached is debug versions of safedexml for future reference if such a bug ever resurfaces again.

I still suspect that the safedexml.fields.List has a minor bug in its implementation, as discussed in the commit messages. However I don't have the time nor the knowledge to investigate further. As far as I can see the project is semi dead, so I don't suspect that it will make any difference trying to make bug reports at the safedexml github page.
This line needs to stay in as it provides backwards compatibility to the older Canari API for a MaltegoTransformRequestMessage. If you take a look below, there is a property called fields which provides access to the first input entity's fields.