input: Write a python function to calculate the average rating for each restaurant from a given CSV file.
  assert calculate_avg_ratings('restaurant_ratings.csv').equals(pd.Series([4.5, 3.2, 4.8], index=['Restaurant A', 'Restaurant B', 'Restaurant C']))
  assert calculate_avg_ratings('empty_file.csv').equals(pd.Series(dtype='float64'))
  assert calculate_avg_ratings('single_restaurant.csv').equals(pd.Series([4.0], index=['Restaurant A']))
output: import pandas as pd
  
  def calculate_avg_ratings(filename):
      df = pd.read_csv(filename)
      if df.empty:
          return pd.Series(dtype='float64')
      avg_ratings = df.groupby('Restaurant')['Rating'].mean()
      return avg_ratings