fixed no join bug when no data in table

csst_dag/dag/_base_dag.py

@@ -88,12 +88,15 @@ class BaseDAG:
             join_type="inner",
         )
         u_data_basis = table.unique(filtered_data_basis["dataset", "obs_id"])
-        filtered_plan_basis = table.join(
-            u_data_basis,
-            plan_basis,
-            keys=["dataset", "obs_id"],
-            join_type="inner",
-        )
+        if len(u_data_basis) == 0:
+            filtered_plan_basis = plan_basis[:0]
+        else:
+            filtered_plan_basis = table.join(
+                u_data_basis,
+                plan_basis,
+                keys=["dataset", "obs_id"],
+                join_type="inner",
+            )
         return filtered_plan_basis, filtered_data_basis
 
     def schedule(