handle setupFS with null user

Signed-off-by: Robin Appelman <robin@icewind.nl>

Author: icewind1991

lib/private/legacy/OC_Util.php

@@ -307,16 +307,16 @@ public static function setupRootFS(string $user = '') {
 	}
 
 	/**
-	 * Can be set up
+	 * Setup the file system
 	 *
-	 * @param string $user
+	 * @param string|null $user
 	 * @return boolean
 	 * @description configure the initial filesystem based on the configuration
 	 * @suppress PhanDeprecatedFunction
 	 * @suppress PhanAccessMethodInternal
 	 */
-	public static function setupFS($user = '') {
-		self::setupRootFS($user);
+	public static function setupFS(?string $user = '') {
+		self::setupRootFS($user ?? '');
 
 		if (self::$fsSetup) {
 			return false;