fix(mcp): address bito review — pre_validate empty-list and chart_type=None error

- pivot_table.pre_validate: `not config.get("rows/metrics")` catches both
  missing keys and empty lists, matching PivotTableChartConfig min_length=1
- chart_utils.map_config_to_form_data: omit `(chart_type=None)` suffix from
  ValueError when chart_type is None to avoid misleading error messages

superset/mcp_service/chart/chart_utils.py

@@ -337,6 +337,8 @@ def map_config_to_form_data(
     plugin = get_registry().get(chart_type) if chart_type else None
 
     if plugin is None:
+        if chart_type is None:
+            raise ValueError(f"Unsupported config type: {type(config)}")
         raise ValueError(
             f"Unsupported config type: {type(config)} (chart_type={chart_type!r})"
         )

superset/mcp_service/chart/plugins/pivot_table.py

@@ -47,9 +47,9 @@ class PivotTableChartPlugin(BaseChartPlugin):
     ) -> ChartGenerationError | None:
         missing_fields = []
 
-        if "rows" not in config:
+        if not config.get("rows"):
             missing_fields.append("'rows' (row grouping columns)")
-        if "metrics" not in config:
+        if not config.get("metrics"):
             missing_fields.append("'metrics' (aggregation metrics)")
 
         if missing_fields: