Return null when findOne could not find record, closes #128

lib/abstract-class.js

@@ -381,7 +381,7 @@ AbstractClass.findOne = function findOne(params, cb) {
     }
     params.limit = 1;
     this.all(params, function (err, collection) {
-        if (err || !collection || !collection.length > 0) return cb(err);
+        if (err || !collection || !collection.length > 0) return cb(err, null);
         cb(err, collection[0]);
     });
 };