input: Write a function to find the sum of two closest values to zero from a given list.
  assert sum_of_two_closest_to_zero([1, 5, -3, 0, -7]) == -3
  assert sum_of_two_closest_to_zero([10, -10, 20, -20, 30, -30]) == 0
  assert sum_of_two_closest_to_zero([1, 2, 3, 4, 5]) == 3
output: def sum_of_two_closest_to_zero(lst):
      if len(lst) < 2:
          return None
      lst.sort(key=abs)
      closest_sum = lst[0] + lst[1]
      return closest_sum