N°2013 Setup : fix crash on compilation if initial DB info were wrong

Use case :
- having in the iTop config file wrong DB parameters (unreachable server for example)
- launch the setup in update mode
- fix the DB config in the wizard

The PR #351 solved the errors that were happening in the following steps.
But we still had an error during compilation, caused by the maintenance mode activation.

core/config.class.inc.php

@@ -2473,6 +2473,8 @@ class Config
 	 * @param string|null $sModulesDir
 	 * @param bool $bPreserveModuleSettings
 	 *
+	 * @return void
+	 *
 	 * @throws \Exception
 	 * @throws \CoreException
 	 */