Report an empty TOML command value as a handled error

[dylanpulver]

install_command = [] in TOML raises AttributeError: 'NoneType' object has no attribute 'args' with a traceback and exit code 2. list_dependencies_command = [] does the same. The ini spelling install_command= reports unable to determine pip install command: attempting to parse '' into a command failed and exits 1.

The cause

Convert.to_command is declared Command | None, and the two loaders disagree on what None means. StrConvert.to_command("") raises ValueError; TomlLoader.to_command([]) returns None, which is what lets #3388 drop empty entries inside commands. For a scalar of_type=Command that None is cast straight through to the caller, which then dereferences it.

The fix

to_command now returns a Command or raises, in every loader. The TOML rule that an empty entry inside a list of commands is dropped moves into TomlLoader.to_list, which is where StrConvert.to_list already drops empty tokens. convert.py loses its Command-shaped branch in the shared list path, so the format-specific rule no longer leaks into the format-agnostic layer.

Making the abstract return type Command also surfaced that StrConvert.to_command never returned None in the first place - ty rejected the override until the annotation was corrected.

Both formats now stop before the dereference:

$ printf '[env.py]\ninstall_command = []\n' > tox.toml && tox r -e py
py: failed to load py.install_command: attempting to parse [] into a command failed

Drive-by: TomlLoader.to_set was a byte-for-byte copy of to_list and now delegates to it.

Verification

test_toml_loader_command_empty_nok covers the scalar rejection and test_toml_loader_command_list_drops_empty pins the #3388 behaviour; removing either half of the change fails the matching test. test_pip_toml_empty_command_error is parametrised over install_command and list_dependencies_command, the two scalar Command settings.

Full suite passes, as do tox -e type (ty, mypy, pyrefly) and tox -e fix.

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation