PDM ignores PDM_PYPI_USERNAME and PDM_PYPI_PASSWORD when there are no defaults in config

[theredfoxlee]

• I have searched the issue tracker and believe that this is not a duplicate.

Description

PDM ignores PDM_PYPI_USERNAME and PDM_PYPI_PASSWORD when there are no defaults in config.

Steps to reproduce

1. Use PDM with version 2.7.0 without any user configuration.
2. Define custom PDM_PYPI_URL environment variable (along with PDM_PYPI_USERNAME and PDM_PYPI_PASSWORD).
3. Try to install any package (it will fail).
4. Add dummy pypi.username and pypi.password to PDM's config.
5. Try to install package from point 3 again (it will succeed to read PDM_PYPI_USERNAME and PDM_PYPI_PASSWORD).

Actual behavior

[janiec@janiec pdm-bug-test]$ pdm --version
PDM, version 2.7.0
[janiec@janiec pdm-bug-test]$ env | grep PDM | cut -d'=' -f1
PDM_PYPI_USERNAME
PDM_PYPI_URL
PDM_PYPI_PASSWORD
[janiec@janiec pdm-bug-test]$ pdm add black
Adding group default to lockfile
Adding packages to default dependencies: black
See /tmp/pdm-lock-rglptr1i.log for detailed debug log.
[PdmException]: The credentials for pkgs.dev.azure.com are not provided. To give them via interactive shell, please rerun the command with `-v` option.
Add '-v' to see the detailed traceback
[janiec@janiec pdm-bug-test]$ pdm config pypi.username foo
WARNING: the config is shadowed by env var 'PDM_PYPI_USERNAME', the value set won't take effect.
[janiec@janiec pdm-bug-test]$ pdm config pypi.password foo
WARNING: the config is shadowed by env var 'PDM_PYPI_PASSWORD', the value set won't take effect.
[janiec@scarlet pdm-bug-test]$ pdm add black
Adding group default to lockfile
Adding packages to default dependencies: black
🔒 Lock successful
Changes are written to pyproject.toml.
Synchronizing working set with lock file: 6 to add, 0 to update, 0 to remove

  ✔ Install packaging 23.1 successful
  ✔ Install click 8.1.3 successful
  ✔ Install black 23.3.0 successful
  ✔ Install platformdirs 3.5.1 successful
  ✔ Install mypy-extensions 1.0.0 successful
  ✔ Install pathspec 0.11.1 successful

🎉 All complete!

Expected behavior

I expected PDM_PYPI_USERNAME and PDM_PYPI_PASSWORD to be read without having to create dummy entries in the PDM configuration. This issue is particularly noticeable in the Dockerfile, where PDM starts with a fresh configuration and is expected to read from private PyPI based on the environment variables.