Ensure null values in @type are removed

[lanthaler]

Since terms can now be explicitly mapped to null, it is possible that values in @type expand to null. These should be removed in expansion:

{
  "@context": {
    "@vocab": "http://xmlns.com/foaf/0.1/",
    "Person": { "@id": null }
  },
  "@id": "http://me.markus-lanthaler.com/",
  "name": "Markus Lanthaler",
  "@type": "Person",
  "Person": "should be removed as well"
}