Cross-section substitution fails since 4.39.0 when there should be an empty string #3809
Description
In the devpi-server tox.ini I'm doing conditional substitution for the command like this:
[tox]
envlist=py39{,-keyfs_sqlite,-hash_hl},py314,pypy3,flake8
[devpisettings]
storagebackend=
keyfs_sqlite: --devpi-server-storage-backend=devpi_server.keyfs_sqlite
storagefsbackend=
hash_hl: --devpi-server-storage-fs-backend=hash_hl
[testenv]
setenv =
LANG = C
commands=
py.test -p pytest_devpi_server --instafail {[devpisettings]storagebackend} {[devpisettings]storagefsbackend} {posargs}
...[remaining snipped]When trying to run the py311-keyfs_sqlite environment the {[devpisettings]storagefsbackend} is kept as is instead of resulting in an empty string.