MAGETWO-75458: [Backport] - Fix overwrite default value image/file with NULL #10253 - for 2.2

Author: VladimirZaets

app/code/Magento/Config/Model/Config/Backend/File.php

@@ -111,6 +111,8 @@ public function beforeSave()
         } else {
             if (is_array($value) && !empty($value['delete'])) {
                 $this->setValue('');
+            } elseif (is_array($value) && !empty($value['value'])) {
+                $this->setValue($value['value']);
             } else {
                 $this->unsValue();
             }