Context. An online store wants to know how many orders each customer placed.
Your task. Write orders_per_customer(df) that returns a Series indexed by customer_id with the count of orders per customer.
Example. Input rows (C1, O1), (C2, O2), (C1, O3) → Series {C1: 2, C2: 1}.
Notes. Order of customers in the result doesn't matter. Empty DataFrame → empty Series.