[#94] Support for cloning an Empty class instance from fast-querystring

[planttheidea]

Reason for change

In #94 , it was identified that cloning an object with a prototype that lacks a constructor will fail.

const Empty = function() {};
Empty.prototype = Object.create(null);

const object = new Empty();
const clone = copy(object);

It was failing on calling Function.prototype.toString on the prototype.constructor, which was null.

Change

Check for the existence of the constructor before calling toString on it. In the above case, it causes the condition to fail, falling back to using Object.create to construct a new object with the Empty.prototype.