fixed no join bug when no data in table

csst_dag/dag/_base_dag.py

@@ -88,6 +88,9 @@ class BaseDAG:
             join_type="inner",
         )
         u_data_basis = table.unique(filtered_data_basis["dataset", "obs_id"])
+        if len(u_data_basis) == 0:
+            filtered_plan_basis = plan_basis[:0]
+        else:
             filtered_plan_basis = table.join(
                 u_data_basis,
                 plan_basis,