text: Title: [BUG] sort_values failed after using dropna
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  **Describe the bug**
  when I try to use sort_values(ignore_index=True) after dropna, it raises TypeError: 
  ```
  a = md.Series([1,3,2,np.nan,np.nan])
  a.dropna().sort_values(ignore_index=True).execute()
  ```
  but I can do it in pandas: 
  ```
  b = pd.Series([1, 3, 2, np.nan, np.nan])
  b.dropna().sort_values(ignore_index=True)
  ```
  
  **To Reproduce**
  To help us reproducing this bug, please provide information below:
  1. Your Python version: 3.8.0
  2. The version of Mars you use: 0.6.11
  3. Versions of crucial packages, such as numpy, scipy and pandas: pandas: 1.1.3
  4. Full stack of the error.
  ```
  ValueError                                Traceback (most recent call last)
  c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\pandas\core\dtypes\common.py in ensure_python_int(value)
      170     try:
  --> 171         new_value = int(value)
      172         assert new_value == value
  
  ValueError: cannot convert float NaN to integer
  
  The above exception was the direct cause of the following exception:
  
  TypeError                                 Traceback (most recent call last)
  <ipython-input-18-f7e878c753c1> in <module>
        1 a = md.Series([1,3,2,np.nan,np.nan])
  ----> 2 a.dropna().sort_values(ignore_index=True).execute()
  
  c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\mars\dataframe\sort\sort_values.py in series_sort_values(series, axis, ascending, inplace, kind, na_position, ignore_index, parallel_kind, psrs_kinds)
      317                              parallel_kind=parallel_kind, psrs_kinds=psrs_kinds,
      318                              output_types=[OutputType.series], gpu=series.op.is_gpu())
  --> 319     sorted_series = op(series)
      320     if inplace:
      321         series.data = sorted_series.data
  
  c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\mars\utils.py in _inner(*args, **kwargs)
      454         def _inner(*args, **kwargs):
      455             with self:
  --> 456                 return func(*args, **kwargs)
      457 
      458         return _inner
  
  c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\mars\dataframe\sort\sort_values.py in __call__(self, a)
       97         assert self.axis == 0
       98         if self.ignore_index:
  ---> 99             index_value = parse_index(pd.RangeIndex(a.shape[0]))
      100         else:
      101             if isinstance(a.index_value.value, IndexValue.RangeIndex):
  
  c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\pandas\core\indexes\range.py in __new__(cls, start, stop, step, dtype, copy, name)
      100             raise TypeError("RangeIndex(...) must be called with integers")
      101 
  --> 102         start = ensure_python_int(start) if start is not None else 0
      103 
      104         if stop is None:
  
  c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\pandas\core\dtypes\common.py in ensure_python_int(value)
      172         assert new_value == value
      173     except (TypeError, ValueError, AssertionError) as err:
  --> 174         raise TypeError(f"Wrong type {type(value)} for value {value}") from err
      175     return new_value
      176 
  
  TypeError: Wrong type <class 'float'> for value nan
  ```
  
  **Expected behavior**
  A clear and concise description of what you expected to happen.
  
  **Additional context**
  Add any other context about the problem here.
  