Handling of empty graph terms

[phochste]

An empty graph term results in an illegal n3 output.

in:

@prefix : <urn:example:> .
@prefix log: <http://www.w3.org/2000/10/swap/log#> .

() log:onNegativeSurface { } .

and const str = await write(quads, { format: 'text/n3', prefixes: {} });

gives

() <http://www.w3.org/2000/10/swap/log#onNegativeSurface> {
    ()   .
    
  } .

[jeswr]

There are a couple of things going on here:

1. We don't have a way of distinguishing between empty graphs and blank nodes in RDF/JS.
2. N3.js incorrectly parses this statment as the following two quads when it should really just be one.

[
      Quad {
        id: '',
        _subject: NamedNode { id: 'http://www.w3.org/1999/02/22-rdf-syntax-ns#nil' },
        _predicate: null,
        _object: null,
        _graph: BlankNode { id: '_:n3-72' }
      },
      Quad {
        id: '',
        _subject: NamedNode { id: 'http://www.w3.org/1999/02/22-rdf-syntax-ns#nil' },
        _predicate: NamedNode {
          id: 'http://www.w3.org/2000/10/swap/log#onNegativeSurface'
        },
        _object: BlankNode { id: '_:n3-72' },
        _graph: DefaultGraph { id: '' }
      }
]

[phochste]

Yeah I saw it. Tried to fix it myself on the airplane and noticed it needs more special treatment.