MAGETWO-71549: Impossible to export Advanced Prices on a medium profile

- fix

Author: Oleksandr Gorkun

app/code/Magento/AdvancedPricingImportExport/Model/Export/AdvancedPricing.php

@@ -304,7 +304,7 @@ private function createExportRow(array $tierPriceData): array
     {
         $exportRow = $this->templateExportData;
         foreach (array_keys($exportRow) as $keyTemplate) {
-            if (!empty($tierPriceData[$keyTemplate])) {
+            if (array_key_exists($keyTemplate, $tierPriceData)) {
                 if (in_array($keyTemplate, $this->_priceWebsite)) {
                     $exportRow[$keyTemplate] = $this->_getWebsiteCode(
                         $tierPriceData[$keyTemplate]
@@ -459,7 +459,7 @@ protected function getTierPrices(array $productLinksIds, $table)
      * @param int $websiteId
      * @return string
      */
-    protected function _getWebsiteCode($websiteId)
+    protected function _getWebsiteCode(int $websiteId): string
     {
         if (!array_key_exists($websiteId, $this->websiteCodesMap)) {
             $storeName = ($websiteId == 0)