fix: clean up config.py unused imports, null-YAML safety, and add non-default roundtrip test

src/autossh_mgr/config.py

@@ -1,7 +1,6 @@
 import os
-from dataclasses import dataclass, asdict, field
+from dataclasses import dataclass, asdict, fields as dc_fields
 from pathlib import Path
-from typing import Optional
 
 import click
 import yaml
@@ -28,9 +27,11 @@ class TunnelConfig:
     ssh_options: str = ""
 
 
-_DEFAULTS = TunnelConfig(
-    name="", host="", user="", local_port=0, remote_port=0
-)
+_OPTIONAL_DEFAULTS = {
+    f.name: f.default
+    for f in dc_fields(TunnelConfig)
+    if f.name in ("port", "identity_file", "local_host", "remote_host", "ssh_options")
+}
 
 
 def ensure_dirs(config_dir: Path) -> None:
@@ -47,7 +48,7 @@ def load_tunnels(config_dir: Path) -> list[TunnelConfig]:
     if not tunnels_file.exists():
         return []
     data = yaml.safe_load(tunnels_file.read_text()) or {}
-    return [TunnelConfig(**entry) for entry in data.get("tunnels", [])]
+    return [TunnelConfig(**entry) for entry in (data.get("tunnels") or [])]
 
 
 def save_tunnels(config_dir: Path, tunnels: list[TunnelConfig]) -> None:
@@ -55,8 +56,8 @@ def save_tunnels(config_dir: Path, tunnels: list[TunnelConfig]) -> None:
     for t in tunnels:
         d = asdict(t)
         # Omit optional fields that are at their default values
-        for key in ("port", "identity_file", "local_host", "remote_host", "ssh_options"):
-            if d[key] == getattr(_DEFAULTS, key):
+        for key, default in _OPTIONAL_DEFAULTS.items():
+            if d[key] == default:
                 del d[key]
         rows.append(d)
     tmp = config_dir / "tunnels.yaml.tmp"