input: Write a python function to read a CSV file and return the top N customers sorted by the amount spent.
  assert top_n_customers('customer_data.csv', 3) == [('C001', 500.0), ('C002', 400.0), ('C003', 300.0)]
  assert top_n_customers('customer_data.csv', 5) == [('C001', 500.0), ('C002', 400.0), ('C003', 300.0), ('C004', 200.0), ('C005', 100.0)]
  assert top_n_customers('customer_data.csv', 0) == []
output: def top_n_customers(file_path, n):
      import csv
      from collections import defaultdict
  
      customer_spendings = defaultdict(float)
  
      with open(file_path, mode='r') as file:
          csv_reader = csv.reader(file)
          next(csv_reader)  # Skip the header row
          for row in csv_reader:
              customer_id, amount_spent = row
              customer_spendings[customer_id] += float(amount_spent)
  
      sorted_customers = sorted(customer_spendings.items(), key=lambda x: x[1], reverse=True)
      return sorted_customers[:n]