Fix pager->current() breaking when no data

[aaron-junot]

Fixes #354

Script:

  // 'account' should be a valid account with canceled subscriptions but no live subscriptions
  $subscriptions = Recurly_SubscriptionList::getForAccount('account', ['state' => 'live']);
  print $subscriptions->count() . "\n";
  $sub = $subscriptions->current();
  var_dump($sub);

Before this commit, this script would throw an exception. Now it prints

0
NULL

[drewish]

Nit pick but since this kind of tags on to the Nested case above what do you think about testFromEmptyNested() or testEmptyNested()?