input: Write a function to compute the sum of two variables and store the result in a third variable. If the given variables are empty or not provided, then return None.
  
  assert compute_sum(5, 8)==13
  assert compute_sum(0, 0)==0
  assert compute_sum(-1, 1)==0
  assert compute_sum(None, None)==None
  assert compute_sum(7, None)==None
output: def compute_sum(a, b):
      if a is None or b is None:
          return None
      return a + b