sort_by("col", nulls_last=True).over("other") does not put nulls at the end

[Alex23rodriguez]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

For example:

df = pl.DataFrame(
    {
        "a": [1, 10, 9, 3, 2, 4, 7, 5, 6, 8, None, None],
        "b": [1, 2, 3, 1, 4, 2, 3, 3, 2, 1, 1, 2],
    }
)

print(df.select(pl.all().sort_by("a", nulls_last=True).over("b")))

gives

shape: (12, 2)
┌──────┬─────┐
│ a    ┆ b   │
│ ---  ┆ --- │
│ i64  ┆ i64 │
╞══════╪═════╡
│ null ┆ 1   │
│ null ┆ 2   │
│ 5    ┆ 3   │
│ 1    ┆ 1   │
│ 2    ┆ 4   │
│ 4    ┆ 2   │
│ 7    ┆ 3   │
│ 9    ┆ 3   │
│ 6    ┆ 2   │
│ 3    ┆ 1   │
│ 8    ┆ 1   │
│ 10   ┆ 2   │
└──────┴─────┘

Log output

No response

Issue description

nulls_last parameter does not put null values at the end

Expected behavior

the desired output should look something like this:

shape: (12, 2)
┌──────┬─────┐
│ a    ┆ b   │
│ ---  ┆ --- │
│ i64  ┆ i64 │
╞══════╪═════╡
│ 4    ┆ 2   │
│ 1    ┆ 1   │
│ 3    ┆ 1   │
│ 6    ┆ 2   │
│ 5    ┆ 3   │
│ 7    ┆ 3   │
│ 10   ┆ 2   │
│ 2    ┆ 4   │
│ 8    ┆ 1   │
│ 9    ┆ 3   │
│ null ┆ 2   │
│ null ┆ 1   │
└──────┴─────┘

Installed versions

--------Version info---------
Polars:               0.20.31
Index type:           UInt32
Platform:             macOS-14.5-arm64-arm-64bit
Python:               3.11.6 (main, Jun  4 2024, 10:11:53) [Clang 15.0.0 (clang-1500.1.0.2.5)]

[mcrumiller]

To clarify, this only occurs when .over(...) is used.

[Alex23rodriguez]

Thanks for pointing this out. I updated the title to reflect this