performance improvements

Get rid of unnecessary reviews queries on product listing:
"select review_entity_summary.* from review_entity_summary where (review_entity_summary.entity_pk_value = ?) and (store_id = ?)" 
This query was executed for each product on listing which didn't have a review summary(@see: Magento\Review\Block\Product\ReviewRenderer::getReviewsSummaryHtml)

Author: apasare

app/code/Magento/Review/Model/Review.php

@@ -5,6 +5,7 @@
  */
 namespace Magento\Review\Model;
 
+use Magento\Framework\DataObject;
 use Magento\Catalog\Model\Product;
 use Magento\Framework\DataObject\IdentityInterface;
 use Magento\Review\Model\ResourceModel\Review\Product\Collection as ProductCollection;
@@ -327,6 +328,9 @@ public function appendSummary($collection)
                     $item->setRatingSummary($summary);
                 }
             }
+            if (!$item->getRatingSummary()) {
+                $item->setRatingSummary(new DataObject());
+            }
         }
 
         return $this;