text: Title: 场外基金调用fund_nav() 缺少资产净值等信息
  Body: 代码如下
  ```
   print(pro.fund_nav(ts_code="000171.OF"))
  ```
  结果如下
  ```
          ts_code  ann_date  end_date  unit_nav  accum_nav accum_div     net_asset  total_netasset  adj_nav update_flag
  0     000171.OF  20200815  20200814     1.959      1.959      None           NaN             NaN    1.959           0
  1     000171.OF  20200814  20200813     1.953      1.953      None           NaN             NaN    1.953           0
  2     000171.OF  20200813  20200812     1.953      1.953      None           NaN             NaN    1.953           0
  3     000171.OF  20200812  20200811     1.961      1.961      None           NaN             NaN    1.961           0
  4     000171.OF  20200811  20200810     1.964      1.964      None           NaN             NaN    1.964           0
  ...         ...       ...       ...       ...        ...       ...           ...             ...      ...         ...
  1696  000171.OF  20130917  20130916     1.007      1.007      None           NaN             NaN    1.007           0
  1697  000171.OF  20130914  20130913     1.005      1.005      None  6.211282e+08    6.211282e+08    1.005           0
  1698  000171.OF  20130907  20130906     1.004      1.004      None  6.205057e+08    6.205057e+08    1.004           0
  1699  000171.OF  20130831  20130830     0.997      0.997      None  6.163435e+08    6.163435e+08    0.997           0
  1700  000171.OF  20130824  20130823     1.000      1.000      None  6.182839e+08    6.182839e+08    1.000           0
  ```
  
  缺少最新的net_asset 和 total_netasset 信息
  https://tushare.pro id: 386529