Fix fatal error when stftime format was null

Before the polyfill, this would only have returned false. So this is a bc fix.

Author: onli

include/functions.inc.php

@@ -140,7 +140,11 @@ function serendipity_strftime($format, $timestamp = null, $useOffset = true, $us
                 } elseif ($useOffset) {
                     $timestamp = serendipity_serverOffsetHour($timestamp);
                 }
-                $out = PHP81_BC\strftime($format, (int)$timestamp);
+                if ($format == null) {
+                    $out = false;
+                } else {
+                    $out = PHP81_BC\strftime($format, (int)$timestamp);
+                }
                 break;
 
             case 'persian-utf8':