MAGETWO-57397: Update gallery entry via API doesn't work - for mainline

Author: Mulderua

app/code/Magento/Catalog/Model/Product/Gallery/GalleryManagement.php

@@ -96,7 +96,9 @@ public function update($sku, ProductAttributeMediaGalleryEntryInterface $entry)
         foreach ($existingMediaGalleryEntries as $key => $existingEntry) {
             if ($existingEntry->getId() == $entry->getId()) {
                 $found = true;
-                $entry->setId(null);
+                if ($entry->getFile()) {
+                    $entry->setId(null);
+                }
                 $existingMediaGalleryEntries[$key] = $entry;
                 break;
             }