url_base_pathname default value is None

Describe your context

The default value for dash.Dash instance attribute url_base_pathname is None instead of / as indicated in the documentation. When deployed on localhost:8050 there are no issues, however when deployed in a different environment (e.g. behind and ingress gateways) things break. url_base_pathname or the env var DASH_URL_BASE_PATHNAME should be actively set to /.

Installed versions:

dash                 2.5.1
dash-core-components 2.0.0
dash-html-components 2.0.0
dash-table           5.0.0

Describe the bug

According to the documentation, the default value for url_base_pathname should be /, however it is None. In the class definition of dash.Dash, we see that the default is url_base_pathname=None.

from dash import Dash

app = Dash(name="foo")

app.config["url_base_pathname"] is None
# True

Expected behavior

The default value for url_base_pathname should default to /.

Screenshots

The behaviour is confirmed in the console of a running app, line 20, "url_base_pathname": null.

Other comments

In the documentation it is stated that Both requests_pathname_prefix and routes_pathname_prefix default to url_base_pathname, however these two are correctly defaulting to /, even though in app.Dash they are set to requests_pathname_prefix=None and routes_pathname_prefix=None respectively.

Continuing the code snippet above

app.config["routes_pathname_prefix"]
# '/'

app.config["requests_pathname_prefix"]
# '/'

Start with the Dash class definition and reproduce the issue's Dash(name="foo") configuration snippet. Compare the documented default for url_base_pathname with the observed config values and verify the related prefix defaults. Done means the default behavior and documentation agree, with regression coverage if the repository provides a relevant test location.