Add empty attributes in xmldom.createDocument

I'm not sure how xmldom is supposed to be used, but right now it's a bit difficult. I wanted to make this work, but had null pointers:
```nimrod
import xmldom

var
  dom = getDOM()
  document = dom.createDocument("", "root")
  topElement = document.documentElement
  firstElement = document.createElement "element"
  textNode = document.createTextNode "Some text here"

topElement.appendChild firstElement
firstElement.appendChild textNode

echo document
```

Many more attributes are set to `nil`, but I don't know what to do about that.