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