Updates the typed properties.

Based on: https://github.com/nextcloud/server/pull/39013#discussion_r1242340826

Co-authored-by: Côme Chilliet <91878298+come-nc@users.noreply.github.com>
Signed-off-by: Faraz Samapoor <fsa@adlas.at>

lib/private/Security/Certificate.php

@@ -32,9 +32,9 @@ use OCP\ICertificate;
 class Certificate implements ICertificate {
 	protected string $name;
 
-	protected mixed $commonName;
+	protected ?string $commonName;
 
-	protected mixed $organization;
+	protected ?string $organization;
 
 	protected $serial;
 
@@ -42,9 +42,9 @@ class Certificate implements ICertificate {
 
 	protected \DateTime $expireDate;
 
-	protected mixed $issuerName;
+	protected ?string $issuerName;
 
-	protected mixed $issuerOrganization;
+	protected ?string $issuerOrganization;
 
 	/**
 	 * @param string $data base64 encoded certificate