Tripod shouldn't allow 'null' values for URIs

[rsinger]

Tripod doesn't (appear) to stop null from being stored in a resource triple, so you can get something like:

{
    "dct:isPartOf": {
        "u": null
    }
}

which causes all kinds of problems in joins for views and table rows, etc. I cannot think of a scenario where this would be desired behavior. I also don't think it should exist for literals, but I'm more certain about resource triples.