Harden Dataflow preview behavior

src/govoplan_dataflow/backend/executor.py

@@ -267,10 +267,10 @@ def _sort_rows(rows: list[dict[str, Any]], config: dict[str, Any]) -> list[dict[
     for field in reversed(config["fields"]):
         column = str(field["column"])
         reverse = field.get("direction", "asc") == "desc"
-        result.sort(
-            key=lambda row: (row.get(column) is None, _sortable_value(row.get(column))),
-            reverse=reverse,
-        )
+        concrete = [row for row in result if row.get(column) is not None]
+        nulls = [row for row in result if row.get(column) is None]
+        concrete.sort(key=lambda row: _sortable_value(row[column]), reverse=reverse)
+        result = [*concrete, *nulls]
     return result