Prevent call of Config.prototype._diffDeep with an undefined argument

`Config.prototype._diffDeep` crashes if called with a value of `undefined` for the `object1` parameter, but the recursive calls of the method could make it happen.

Example code which would have caused a crash :
`var obj1 = {a: {}};
var obj2 = {a: {b: {c: 'd'}}};
t._diffDeep(obj1, obj2);`