Fix cache bypass

If the contact is not impacted by any acls then ids will be an empty array. However, currently
an empty array is treated as 'uncached' rather than 'cached by empty' which
would be represented by null

CRM/ACL/BAO/ACL.php

@@ -480,7 +480,7 @@ SELECT g.*
       $cacheKey = CRM_Utils_Cache::cleanKey("$tableName-$aclKeys");
       $cache = CRM_Utils_Cache::singleton();
       $ids = $cache->get($cacheKey);
-      if (!$ids) {
+      if (!is_array($ids)) {
         $ids = [];
         $query = "
 SELECT   a.operation, a.object_id