input: Write a function that takes a string containing a numerical expression and returns the result. If the input string is empty, return None.
  assert eval_expr("3 + 5 * 2") == 13
  assert eval_expr("10 / 2") == 5.0
  assert eval_expr("7 - 3") == 4
  assert eval_expr("") == None
  assert eval_expr("12") == 12
output: def eval_expr(s):
      if not s:
          return None
      return eval(s)