Return false for an empty array in PPArrayUtil::isAssocArray()

Today i had a problem using the PayPal PHP Rest Api. When the PayPal server returns an json string with an empty array for related_resources, the SDK returned an uninitialized object instead of an empty array.

For example
```php
$json = '{"id":"PAY-DUMMY","create_time":"2014-10-03T05:05:31Z","update_time":"2014-10-03T05:05:31Z","state":"created","intent":"sale","payer":{"payment_method":"paypal","payer_info":{"shipping_address":{}}},"transactions":[{"amount":{"total":"82.55","currency":"EUR","details":{"subtotal":"66.71","tax":"13.34","shipping":"2.50"}},"item_list":{"items":[{"name":Dummy","sku":"2","price":"64.92","currency":"EUR","quantity":"1"}]},"related_resources":[]}],"links":[{"href":"https://api.paypal.com/v1/payments/payment/PAY-DUMMY","rel":"self","method":"GET"},{"href":"https://www.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=EC-DUMMY","rel":"approval_url","method":"REDIRECT"},{"href":"https://api.paypal.com/v1/payments/payment/PAY-DUMMY/execute","rel":"execute","method":"POST"}]}';

$m = new PayPal\Api\Payment();
$transactions = $m->getTransactions();
$related_resources = $transactions[0]->getRelatedResources();
$isEmpty = is_empty($related_resouces); // expected to be false
```
In this case the $related_resources was not empty. But contained an uninitialized instance of RelatedResources which was created here: https://github.com/paypal/sdk-core-php/blob/master/lib/common/PPModel.php#L51