fix missing database check when doing RL without a given database

This fixes commands like:
assume -s example_02a -c tiny

assume/common/outputs.py

@@ -568,9 +568,9 @@ def get_sum_reward(self):
         query = text(
             f"select unit, SUM(reward) FROM rl_params where simulation='{self.simulation_id}' GROUP BY unit"
         )
-
-        with self.db.begin() as db:
-            rewards_by_unit = db.execute(query).fetchall()
+        if self.db is not None:
+            with self.db.begin() as db:
+                rewards_by_unit = db.execute(query).fetchall()
 
         # convert into a numpy array
         rewards_by_unit = [r[1] for r in rewards_by_unit]