convert issuer to string prevents php 8.1 errors

By forcing $this->issuer to be string, even if null is set, it prevents throwing errors in PHP 8.1 is `rawurlencode` is not allowed to have null as parameter.

It would be better to force string to be already in `__construct`, but this may create a breaking change for existing users.

Author: brainfoolong

lib/TwoFactorAuth.php

@@ -272,7 +272,7 @@ public function getQRText($label, $secret)
     {
         return 'otpauth://totp/' . rawurlencode($label)
             . '?secret=' . rawurlencode($secret)
-            . '&issuer=' . rawurlencode($this->issuer)
+            . '&issuer=' . rawurlencode((string)$this->issuer)
             . '&period=' . intval($this->period)
             . '&algorithm=' . rawurlencode(strtoupper($this->algorithm))
             . '&digits=' . intval($this->digits);