`failed to read config file None` when `DEFAULT_CONFIG_FILE` is corrupt

[kurtmckee]

Issue

When DEFAULT_CONFIG_FILE is corrupted, tox outputs failed to read config file None.

Environment

Provide at least:

• OS: Linux

Output of pip list of the host Python, where tox is installed

$ pip list
Package       Version                          Editable project location
------------- -------------------------------- -------------------------
cachetools    6.2.0
chardet       5.2.0
colorama      0.4.6
distlib       0.4.0
filelock      3.19.1
packaging     25.0
pip           25.2
platformdirs  4.4.0
pluggy        1.6.0
pyproject-api 1.9.1
setuptools    80.9.0
tox           4.30.3.dev0+g5e0784a7f.d20250917 /home/kurt/dev/pr-tox
virtualenv    20.34.0
wheel         0.45.1

Output of running tox

Output of tox -rvv

$ tox --help
ERROR:root:failed to read config file None because File contains no section headers.
file: '/home/kurt/.config/tox/config.ini', line: 1
'[tox\n'
ROOT: failed to read config file None because File contains no section headers.
file: '/home/kurt/.config/tox/config.ini', line: 1
'[tox\n'
usage: tox [-h] [--colored {yes,no}] [--stderr-color {BLACK,BLUE,CYAN,GREEN,LIGHTBLACK_EX,LIGHTBLUE_EX,LIGHTCYAN_EX,LIGHTGREEN_EX,LIGHTMAGENTA_EX,LIGHTRED_EX,LIGHTWHITE_EX,LIGHTYELLOW_EX,MAGENTA,RED,RESET,WHITE,YELLOW}] [-v | -q]
...

Minimal example

$ echo '[tox' > ~/.config/tox/config.ini

[kurtmckee]

I'm working to address this now that #3612 has merged. The fix is trivial but I need to add a test before opening a PR.