Mapper/Module/Schema fixes thanks to new tests

Models/InvoiceMapper.php

@@ -0,0 +1,29 @@
+<?php
+/**
+ * Orange Management
+ *
+ * PHP Version 7.2
+ *
+ * @package    Modules\Billing
+ * @copyright  Dennis Eichhorn
+ * @license    OMS License 1.0
+ * @version    1.0.0
+ * @link       http://website.orange-management.de
+ */
+declare(strict_types=1);
+
+namespace Modules\Billing\Models;
+
+use phpOMS\DataStorage\Database\DataMapperAbstract;
+
+/**
+ * Mapper class.
+ *
+ * @package    Modules\Billing
+ * @license    OMS License 1.0
+ * @link       http://website.orange-management.de
+ * @since      1.0.0
+ */
+final class InvoiceMapper extends DataMapperAbstract
+{
+}