hotline #66376 test fix

library/ZendAfi/View/Helper/LibraryOpenings.php

@@ -71,9 +71,11 @@ class ZendAfi_View_Helper_LibraryOpenings extends ZendAfi_View_Helper_BaseHelper
 
 
   protected function _renderPeriods($periodical) {
-    return implode('',
-                   array_map([$this, '_renderPeriod'],
-                             $periodical));
+    return $periodical
+      ? implode('',
+                array_map([$this, '_renderPeriod'],
+                          $periodical))
+      : '';
   }