Add warning if the basket is empty for a connector

shopping_client/shopping_client.py

@@ -149,6 +149,8 @@ class shopping_client:
 
                 if len(items):
                     converted_basket[connector.name] = pd.concat(items, axis=1)
+                else:
+                    warn(f"Connector {connector.name} specified but no data found for it in shopping basket. Result will not contain an entry named {connector.name}.")
 
             return {
                 name: data.to_frame().T if data.ndim < 2 else data.T