Allow django.urls.include without a namespace
Allow django.urls.include without a namespace

`URLResolver.namespace` is set to None by default, which raises `TypeError` for `':'.join(namespaces)`. This breaks any apps or includes not using namespaces (like `django.contrib.admin`).

Example:
```
urlpatterns = [
    distill_path('', views.index, name='index', distill_func=lambda: None),
    path('other/', include([
        path('', views.other, name='other'),
    ])),
]
```
mgrp/django-distill