Fix Issue #223 - Implicit fetch cursor returns empty last value

Summary:
#223
When using Implicit Fetch the last object returned
by Cursor is an empty, inexistant object.
This was caused by a very subtle interaction with isJsonObject method.

Test Plan:
Sample code to detect error:

  use FacebookAds\Logger\CurlLogger;
  use FacebookAds\Object\AdAccount;
  use FacebookAds\Cursor;

  Api::instance()->setLogger(new CurlLogger());

  Cursor::setDefaultUseImplicitFetch(true);

  $result = array();
  $params = array();
  $fields = array(
      \FacebookAds\Object\Fields\AdFields::ID,
      \FacebookAds\Object\Fields\AdFields::NAME,
  );

  $account = new AdAccount($account_id);
  $ads = $account->getAds($fields, $params);
  //$ads->setUseImplicitFetch(true);
  foreach ($ads as $ad) {
      $row = [];
      foreach ($fields as $field) {
          $row[$field] = $ad->$field;
      }
      $result[] = $row;
  }

  var_dump($result);

src/FacebookAds/Cursor.php

@@ -163,6 +163,11 @@ private function isJsonObject($object) {
       return false;
     }
 
+    // Consider an empty array as not object
+    if (empty($object)) {
+      return false;
+    }
+
     // A json object is represented by a map instead of a pure list
     return array_keys($object) !== range(0, count($object) - 1);
   }