Solve search with parameter single=true and no results

[estercolero82]

When doing a defiant.search with the option single=true and no results found throws an exception: xres[i] is null. This commit solves the problem.
Sample code:
(async () => {
// import 'defiant'
var defiant = await fetchScript('/res/js/modules/defiant.js');
var data = {
"car": [
{"id": 10, "color": "silver", "name": "Volvo"},
{"id": 11, "color": "red", "name": "Saab"},
{"id": 12, "color": "red", "name": "Peugeot"},
{"id": 13, "color": "yellow", "name": "Porsche"}
],
"bike": [
{"id": 20, "color": "black", "name": "Cannondale"},
{"id": 21, "color": "red", "name": "Shimano"}
]
};

//Search with 0 results looking for a single node
var search = defiant.search(data, '//car[color="brown"]/name', true);
console.log(search);
})();