[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)
in
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 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.

Start in mars/dataframe/sort/sort_values.py, at the Series sort_values path that builds a RangeIndex when ignore_index is true. Reproduce the reported dropna().sort_values(ignore_index=True) case and inspect the index shape or value passed to pandas. Done means the operation executes without TypeError and produces the expected reset index, with a regression test for the example.

Written by the indexing model from the issue text.