fix: [S/MIME] don't sign e-mails if no signing key is set

- fixes e-mails not going out on instances where no signing key was provided

app/Lib/Tools/SendEmail.php

@@ -446,7 +446,9 @@ class SendEmail
         }
 
         if (!$canEncryptGpg && $canEncryptSmime) {
-            $this->signBySmime($email);
+            if (!empty(Configure::read('SMIME.cert_public_sign')) && !empty(Configure::read('SMIME.key_sign'))) {
+                $this->signBySmime($email);
+            }
             $this->encryptBySmime($email, $user['User']['certif_public']);
             $encrypted = true;
         }