fix(config): let empty factor fallbacks override TOML group defaults (tox-dev#4072)

Author: glaziermag

docs/changelog/4072.bugfix.rst

@@ -0,0 +1 @@
+Preserve an explicitly empty ``{factor:label:}`` fallback instead of using the declared factor group default.

src/tox/config/loader/replacer.py

@@ -334,7 +334,7 @@ def replace_factor(conf: Config, args: list[str], conf_args: ConfigLoadArgs) ->
     for value in group.values:
         if value in env_factors:
             return value
-    return default or group.default or ""
+    return default if len(args) > 1 else group.default or ""
 
 
 __all__ = [

tests/config/source/test_toml_env_base.py

@@ -579,6 +579,7 @@ def test_env_list_product_labeled_range_factor_group(tox_project: ToxProjectCrea
         pytest.param("task-django42", "django42", id="active-factor-wins"),
         pytest.param("other", "django50", id="group-default"),
         pytest.param("inline", "django42", id="inline-default-wins"),
+        pytest.param("empty-inline", "", id="empty-inline-default-wins"),
     ],
 )
 def test_env_base_factor_group_default(tox_project: ToxProjectCreator, env: str, expected: str) -> None:
@@ -595,6 +596,9 @@ def test_env_base_factor_group_default(tox_project: ToxProjectCreator, env: str,
 
             [env.inline]
             description = "Test {factor:django_version:django42}"
+
+            [env.empty-inline]
+            description = "Test {factor:django_version:}"
         """),
     })
     outcome = project.run("c", "-e", env, "-k", "description")