If the title was 'true' or 'false', the export failed, because we are tying to
concat bool and str. TypeError: cannot concatenate 'str' and 'bool' objects

How to reproduce, install sale, be sure to have one order with delivered = True
In Reporting / Sale analysis, add a group by 'shipped' filed to have 'true' as
title.

This commit closes issue odoo/odoo#13425