Ignore NaN correctly in .quantile

[mroeschke]

Description

From an offline conversation, fixes the follow discrepancy between cudf and pandas

In [1]: import cudf

In [2]: import numpy as np

In [3]: ser = cudf.Series([np.nan, np.nan, 0.9], nan_as_null=False)

In [4]: ser
Out[4]: 
0    NaN
1    NaN
2    0.9
dtype: float64

In [5]: ser.quantile(0.9)
Out[5]: np.float64(nan)

In [6]: import pandas as pd

In [7]: ser = pd.Series([np.nan, np.nan, 0.9])

In [8]: ser.quantile(0.9)
Out[8]: np.float64(0.9)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mroeschke]

/merge