N-Quads serializer cannot deal with default graph.

[RinkeHoekstra]

The following valid N-Quad does not contain a graph:

_:b0 <https://example.com/hasExampleProperty> "some literal value" .

This is correctly parsed into a $rdf.Store.

When iterating over the statement, the graph term looks something like:

{
    "termType": "DefaultGraph",
    "value": "",
    "uri": "chrome:theSession"
}

However, the atomicTermToN3 function in serializer.js does not deal with nodes of type DefaultGraph:

rdflib.js/src/serializer.js

Lines 522 to 567 in c14dfd5

	atomicTermToN3 (expr, stats) {
	switch (expr.termType) {
	case 'BlankNode':
	case 'Variable':
	return expr.toNT()
	case 'Literal':
	var val = expr.value
	if (typeof val !== 'string') {
	throw new TypeError('Value of RDF literal node must be a string')
	}
	// var val = expr.value.toString() // should be a string already
	if (expr.datatype && this.flags.indexOf('x') < 0) { // Supress native numbers
	switch (expr.datatype.uri) {
	case 'http://www.w3.org/2001/XMLSchema#integer':
	return val
	case 'http://www.w3.org/2001/XMLSchema#decimal': // In urtle must have dot
	if (val.indexOf('.') < 0) val += '.0'
	return val
	case 'http://www.w3.org/2001/XMLSchema#double': {
	// Must force use of 'e'
	const eNotation = val.toLowerCase().indexOf('e') > 0;
	if (val.indexOf('.') < 0 && !eNotation) val += '.0'
	if (!eNotation) val += 'e0'
	return val
	}
	case 'http://www.w3.org/2001/XMLSchema#boolean':
	return expr.value === '1' ? 'true' : 'false'
	}
	}
	var str = this.stringToN3(expr.value)
	if (expr.language) {
	str += '@' + expr.language
	} else if (!expr.datatype.equals(this.xsd.string)) {
	str += '^^' + this.atomicTermToN3(expr.datatype, stats)
	}
	return str
	case 'NamedNode':
	return this.symbolToN3(expr)
	default:
	throw new Error('Internal: atomicTermToN3 cannot handle ' + expr + ' of termType: ' + expr.termType)
	}
	}

And fails with the error:

Error: Internal: atomicTermToN3 cannot handle DefaultGraph of termType: DefaultGraph

This should be fixed in atomicTermToN3 function in serializer.js by returning an empty string when the term type is DefaultGraph.

NB this is consistent also with how TriG represents default graphs.