Fix is_mandatory checkbox not saving when unchecked

includes/Admin/Admin.php

@@ -1377,6 +1377,7 @@ class Admin {
 								<th scope='row'><?php echo esc_html__( 'Position', KGVVM_TEXT_DOMAIN ); ?></th>
 								<td>
 									<label>
+										<input type='hidden' name='is_mandatory' value='0' />
 										<input type='checkbox' name='is_mandatory' value='1' <?php checked( $cost ? (bool) $cost->is_mandatory : true, true ); ?> />
 										<?php echo esc_html__( 'Verpflichtende Position', KGVVM_TEXT_DOMAIN ); ?>
 									</label>