Update Stock.php

Relates to issue: magento#8566

A flag set to null will always overwrite the isShowOutOfStock variable. So you can't set the default value from the admin configuration.

app/code/Magento/CatalogInventory/Helper/Stock.php

@@ -156,7 +156,7 @@ public function addIsInStockFilterToCollection($collection)
             $resource = $this->getStockStatusResource();
             $resource->addStockDataToCollection(
                 $collection,
-                !$isShowOutOfStock && $collection->getFlag('require_stock_items')
+                !$isShowOutOfStock || $collection->getFlag('require_stock_items')
             );
             $collection->setFlag($stockFlag, true);
         }