Handle case of "i:nil="true" for the value being null.

[robblovell]

A standard way of handling null in an XML is to do this:

<thing i:nil="true"></thing>

which translates to:

{ "thing": null }

This is different than just having an empty clause:

<thing></thing>

which translates to:

{ "thing":"" }

[robblovell]

Background: http://stackoverflow.com/questions/463597/what-does-inil-true-mean