Regression in 76b993a117b61c41584e95149a67d8a1e9f49dd1.
Thanks Claude Paroz for the report and Carlton Gibson for reviews.
Backport of 82a88d2f48e13ef5d472741d5ed1c183230cfe4c from master
Previous behavior was inconsistent with django.urls.reverse() and
caused that translate_url() created an incorrect URL when an optional
parameter was missing.
When using include() without namespaces of some urlpatterns that
have an include() with namespace, the converters of the parent
include() weren't being used to convert the arguments of reverse().
Thanks Aymeric Augustin for shepherding the DEP and patch review.
Thanks Marten Kenbeek and Tim Graham for contributing to the code.
Thanks Tom Christie, Shai Berger, and Tim Graham for the docs.