config: T8858: fix mutable default argument in config API methods

Four Config class methods used `default=[]` as a parameter default. The
empty list is created once at function-definition time and shared across
all calls; any mutation of the default leaks across callers. This is
mitigated in practice by `_make_path` returning `(self._level + path)`
(which creates a fresh list via `+` rather than mutating via `.append()`),
but the defensive `default=None` + explicit guard is preserved as an
additional safeguard.

Methods touched:
- return_values
- list_nodes
- return_effective_values
- list_effective_nodes

Also document the `default` parameter in each method's docstring `Args:`
section and the corresponding `Returns:` clarifications.

0 files changed, 0 insertions, 0 deletions