Repository: readthedocs/sphinx-notfound-page

Issue:
Disabling prefix leads to "WARNING: The config value `notfound_urls_prefix' has type `NoneType', defaults to `str'."
**Python version:** 3.9.0
**Sphinx version:** 1.8.5
**sphinx-notfound-page version:** 0.6

Here's the relevant part of my `conf.py`:

```python
# sphinx-notfound-page
# https://github.com/readthedocs/sphinx-notfound-page
notfound_context = {
    "title": "Page Not Found",
    "body": """
<h1>Page Not Found</h1>
<p>Sorry, we couldn"t find that page.</p>
<p>Try using the search box or go to the homepage.</p>
""",
}

# on_rtd is whether we are on readthedocs.org, this line of code grabbed from docs.readthedocs.org
on_rtd = os.environ.get("READTHEDOCS", None) == "True"

# Don't add `/en/latest` prefix during local development.
# This makes it easier to test the custom 404 page by loading `/404.html`
# on a local web server.
if not on_rtd:
    notfound_urls_prefix = None
```

I've been able to avoid this by setting the value to `''` (empty string) instead, but the documentation doesn't mention this. I guess we should just update the documentation to reflect this?

Discussion:

Patch:
diff --git a/notfound/extension.py b/notfound/extension.py
index 1520452..b2e276d 100644
--- a/notfound/extension.py
+++ b/notfound/extension.py
@@ -299,6 +299,7 @@ def setup(app):
             default_version=default_version,
         ),
         'html',
+        types=[str, type(None)],
     )
 
     app.connect('config-inited', validate_configs)


Test patch:
diff --git a/tests/test_urls.py b/tests/test_urls.py
index fd9e532..80a2e73 100644
--- a/tests/test_urls.py
+++ b/tests/test_urls.py
@@ -196,6 +196,8 @@ def test_urls_prefix_setting_none(app, status, warning):
     for chunk in chunks:
         assert chunk in content
 
+    assert "The config value `notfound_urls_prefix' has type `NoneType', defaults to `str'" not in warning.getvalue()
+    assert "build succeeded." in status.getvalue()
 
 @pytest.mark.sphinx(
     srcdir=srcdir,