fix dataframe warning

Author: harisang

src/fetch/orderbook.py

@@ -88,7 +88,14 @@ def get_order_rewards(cls, block_range: BlockRange) -> DataFrame:
 
         # Solvers do not appear in both environments!
         assert set(prod.solver).isdisjoint(set(barn.solver)), "solver overlap!"
-        return pd.concat([prod, barn])
+        if not prod.empty and not barn.empty:
+            return pd.concat([prod, barn])
+        elif not prod.empty:
+            return prod.copy()
+        elif not barn.empty:
+            return barn.copy()
+        else:
+            return pd.DataFrame()
 
     @classmethod
     def get_batch_rewards(cls, block_range: BlockRange) -> DataFrame:
@@ -116,4 +123,11 @@ def get_batch_rewards(cls, block_range: BlockRange) -> DataFrame:
 
         # Solvers do not appear in both environments!
         assert set(prod.solver).isdisjoint(set(barn.solver)), "solver overlap!"
-        return pd.concat([prod, barn])
+        if not prod.empty and not barn.empty:
+            return pd.concat([prod, barn])
+        elif not prod.empty:
+            return prod.copy()
+        elif not barn.empty:
+            return barn.copy()
+        else:
+            return pd.DataFrame()