some fixes and improvements

python_app/models/run_config_codec.py

@@ -24,10 +24,22 @@ def _as_dict(value: Any, context: str) -> dict[str, Any]:
     return value
 
 
+def _read_str(payload: dict[str, Any], key: str, default: str) -> str:
+    """Return payload string, treating an explicit JSON `null` as missing.
+
+    `payload.get(key, default)` returns `None` when the key exists with value
+    `null`, which is then coerced into the literal string `"None"` by `str()`.
+    """
+    value = payload.get(key, default)
+    if value is None:
+        return default
+    return str(value)
+
+
 def _load_preprocess_asset(payload: dict[str, Any], target: PreprocessAssetModel) -> None:
     """Load preprocess asset fields into target model."""
-    target.set_name = str(payload.get("set_name", target.set_name))
-    target.bundle_path = str(payload.get("bundle_path", target.bundle_path))
+    target.set_name = _read_str(payload, "set_name", target.set_name)
+    target.bundle_path = _read_str(payload, "bundle_path", target.bundle_path)
 
 
 def _load_string_list(payload: dict[str, Any], key: str, context: str) -> list[str]: