LIMIT 0 should be preserved

[QuarksToQuasars]

https://www.w3.org/TR/sparql11-query/#modResultLimit

A LIMIT of 0 would cause no results to be returned. A limit may not be negative.

    var parser = new SparqlParser();
    var parsedQuery = parser.parse('SELECT * WHERE { ?s ?p ?o. } LIMIT 0');
    var generator = new SparqlGenerator();
    var generatedQuery = generator.stringify(parsedQuery);
    var expectedQuery =
      'SELECT * WHERE { ?s ?p ?o. }\n' +
      'LIMIT 0';
    expect(generatedQuery).toEqual(expectedQuery);
  });

fails as the generator omits the limit in case of 0 (https://github.com/RubenVerborgh/SPARQL.js/blob/main/lib/SparqlGenerator.js#L79)

[QuarksToQuasars]

Was merged with #184 .

Thanks @RubenVerborgh !