[data] sort() does not work with string column with None

[wingkitlee0]

What happened + What you expected to happen

When applying sort() on a string column with None, it will raise the following error

TypeError: '<' not supported between instances of 'str' and 'NoneType'

Versions / Dependencies

In [21]: ray.__version__
Out[21]: '2.39.0'

Reproduction script

In [17]: df = pd.DataFrame({"x": [None, None, "a", "c", None]})

In [18]: df.sort_values(by="x")
Out[18]:
      x
2     a
3     c
0  None
1  None
4  None

In [19]: ds = ray.data.from_pandas(df)

In [20]: ds.sort("x").take_all()

gives an error. However, pandas Dataframe works.

Issue Severity

Medium: It is a significant difficulty but I can work around it.

[alexeykudinkin]

This should be fixed by #48750