Relative URLs in an `@list` can produce bad RDF

[dlongley]

Because the _objectToRDF returns null for objects that are relative URLs, and the following code within _listToRDF does not check for this, it's possible that quads are added to the dataset that have null objects, which is invalid. Checks for null objects need to be added here:

https://github.com/digitalbazaar/jsonld.js/blob/master/lib/toRdf.js#L169
https://github.com/digitalbazaar/jsonld.js/blob/master/lib/toRdf.js#L188

To avoid later potential crashes/problems when the dataset is processed (by whatever code).