input: Write a function to evaluate a numerical expression given as a string and return the result. If the input string is empty, return None.
  assert eval_expr("5+3*2")==11
  assert eval_expr("7-4/2")==5
  assert eval_expr("10*2+3")==23
  assert eval_expr("6/3-1")==1
  assert eval_expr("")==None
output: def eval_expr(s):
      if not s:
          return None
      return eval(s)