MC-19661: Export Products bug. I can't exclude attributes from CSV file.

Author: Roman Lytvynenko

app/code/Magento/CatalogImportExport/Model/Export/Product.php

@@ -724,7 +724,9 @@ public function _getHeaderColumns()
      */
     protected function setHeaderColumns($customOptionsData, $stockItemRows)
     {
-        $exportAttributes = count($this->_parameters["skip_attr"]) ?
+        $exportAttributes = (
+            array_key_exists("skip_attr", $this->_parameters) && count($this->_parameters["skip_attr"])
+        ) ?
             array_intersect($this->_getExportMainAttrCodes(), $this->_getExportAttrCodes()) :
             $this->_getExportMainAttrCodes();