use empty string comparison instead of strlen call

Author: xHeaven

lib/TwoFactorAuth.php

@@ -239,7 +239,7 @@ private function getTimeSlice(?int $time = null, int $offset = 0): int
 
     private function base32Decode(string $value): string
     {
-        if (strlen($value) == 0) {
+        if ($value === '') {
             return '';
         }