Check isset on CoveredRateCenter key

Certain states do not have any rate centers available and thus this returns nothing.

You can check from:
* NH
* NL
* QC
* NU

Author: tm1000

src/CoveredRateCenter.php

@@ -13,7 +13,7 @@ public function getList($filters = Array()) {
         $rcs = [];
         $data = parent::_get('coveredRateCenters', $filters, Array("page"=> 1, "size" => 30), Array("page", "size"));
 
-        if($data['CoveredRateCenter']) {
+        if(isset($data['CoveredRateCenter'])) {
             $items =  $data['CoveredRateCenter'];
 
             if($this->is_assoc($items))