Fixed when LoadPage returns an empty array

When the LoadPage is called by the Count property, which happens via the
this[0] invocation,
and the LoadPage  returns an empy JToken[], we were getting an
exception, since the indexer (this[i]) method doesn't check if it is an
empty array.

Here is the typical consumer code

var client = new UserCoice.Client(subdomain, key, secret);
client.LoginAsOwner();
var result = client.GetCollection(pathToCollection);
Console.WriteLine("Num of Items: " + result.Count);

If it is an empty collection, without this fix, we get an index out of
range exception.

Here is a json response that will cause the exception

{"response_data":{"page":1,"per_page":100,"total_records":0,"filter":"all","sort":"newest"},
"manual_actions":[]}

This is a real example, obtained using the manual_actions method (GET
/api/v1/manual_actions)