Fix some warnings:

Warning: Illegal string offset 'active' in /wordpress/wp-content/plugins/pronamic-google-maps/classes/Pronamic/Google/Maps/Admin.php on line 73 (line 73: $types = $options['active'];
Warning: Illegal string offset 'active' in  /wordpress/wp-content/plugins/pronamic-google-maps/classes/Pronamic/Google/Maps/Maps.php on line 398 (line 398: $activeTypes = $options['active'];