input: Write a function to calculate the average of a list of numbers. If the list is empty return None.
  assert avg_list([1, 2, 3, 4, 5])==3.0
  assert avg_list([10, 20, 30, 40, 50])==30.0
  assert avg_list([0, 0, 0, 0])==0.0
  assert avg_list([1])==1.0
  assert avg_list([])==None
output: def avg_list(numbers):
      if not numbers:
          return None
      return sum(numbers) / len(numbers)