input: Write a function to calculate the average of five numbers entered by a user. The function should prompt the user to input five numbers, store them in a list, calculate their average, and then print the result. If no numbers are entered, it should return None.
  
  
  assert calculate_average() == None
  assert calculate_average([10, 20, 30, 40, 50]) == 30.0
  assert calculate_average([0, 0, 0, 0, 0]) == 0.0
  assert calculate_average([-10, -20, -30, -40, -50]) == -30.0
  
output: def calculate_average(numbers=None):
      if numbers is None:
          numbers = []
          for i in range(5):
              num = float(input("Enter a number: "))
              numbers.append(num)
      if not numbers:
          return None
      return sum(numbers) / len(numbers)