use constant as default value in value getter

Signed-off-by: Bennet Becker <dev@bennet.cc>

lib/private/Mail/Message.php

@@ -337,6 +337,6 @@ class Message implements IMessage {
 		$headers = $this->swiftMessage->getHeaders();
 
 		return $headers->has('Auto-Submitted') ?
-			$headers->get('Auto-Submitted')->toString() : "no";
+			$headers->get('Auto-Submitted')->toString() : AutoSubmittedValue::NO;
 	}
 }

lib/public/Mail/AutoSubmittedValue.php

@@ -31,7 +31,7 @@ namespace OCP\Mail;
  * The value "auto-notified" as per RFC 5436 is deliberately omitted as it is
  * meant of notification of the sieve system.
  *
- * @see https://www.iana.org/assignments/auto-submitted-keywords/auto-submitted-keywords.xhtml
+ * @link https://www.iana.org/assignments/auto-submitted-keywords/auto-submitted-keywords.xhtml
  *
  * @since 26.0.0
  */