feature/M095M01A-29 fastly#23: [MAGENTO 2] Make sure we are not sending purges for empty surrogate keys

- Use array_filter to remove any `falsy` elements for surrogate keys array before purge
- Return early if surrogate keys array is empty

Author: MartinPeverelli

Diff for: Model/Api.php

@@ -179,6 +179,11 @@ public function cleanUrl($url)
      */
     public function cleanBySurrogateKey($keys)
     {
+        $keys = array_filter($keys);
+        if (empty($keys)) {
+            return true;
+        }
+
         $type = 'clean by key on ';
         $uri = $this->_getApiServiceUri() . 'purge';
         $num = count($keys);