Return an empty list for an empty env in _env_names

Check `env is None` so an empty dict yields `[]` rather than the dict,
honoring the list[str] | None return contract.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: anticomputer

src/seclab_taskflow_agent/mcp_utils.py

@@ -179,7 +179,7 @@ def _env_names(env: dict[str, Any] | None) -> list[str] | None:
     their values must never be written to logs. Callers log the names to keep
     debug output useful without leaking secrets.
     """
-    return sorted(env) if env else env
+    return sorted(env) if env is not None else None
 
 
 def mcp_client_params(

tests/test_mcp_utils.py

@@ -12,7 +12,12 @@
 
 import pytest
 
-from seclab_taskflow_agent.mcp_utils import MCPNamespaceWrap, compress_name, mcp_client_params
+from seclab_taskflow_agent.mcp_utils import (
+    MCPNamespaceWrap,
+    _env_names,
+    compress_name,
+    mcp_client_params,
+)
 
 
 class _FakeTool:
@@ -183,3 +188,11 @@ def test_mcp_client_params_logs_env_names_not_secret_values(monkeypatch, caplog)
     # Redaction is log-only: the real env (with the resolved secret) is still
     # passed through to the MCP server.
     assert params["pkg.tb"][0]["env"]["GH_TOKEN"] == sentinel
+
+
+def test_env_names_returns_sorted_names_or_none():
+    # Non-empty env -> sorted names; empty dict -> empty list; None -> None
+    # (honouring the list[str] | None contract, never leaking values).
+    assert _env_names({"B_VAR": "x", "A_VAR": "{{ env('A') }}"}) == ["A_VAR", "B_VAR"]
+    assert _env_names({}) == []
+    assert _env_names(None) is None