rank().over() replaces wrongly assigns rank=1 to null values in the case that the over() group has a size of 1.

[bfeif]

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

df = (
    pl.DataFrame({
        "a": [2.1, 4, None, 2.5],
        "b": ["j", "j", "k", "l"]
    })
    .with_columns(
        pl.col("a").rank().over("b")
        .name.suffix("_rank_over_b")
    )
)
print(df)

Log output

shape: (4, 3)
┌──────┬─────┬────────┐
│ a    ┆ b   ┆ a_rank │
│ ---  ┆ --- ┆ ---    │
│ f64  ┆ str ┆ f64    │
╞══════╪═════╪════════╡
│ 2.1  ┆ j   ┆ 1.0    │
│ 4.0  ┆ j   ┆ 2.0    │
│ null ┆ k   ┆ 1.0    │
│ 2.5  ┆ l   ┆ 1.0    │
└──────┴─────┴────────┘

### Issue description

Title explains everything.

### Expected behavior

Behavior should be as it is normally when the `over()` group has a size larger than 1:

```python
df = (
    pl.DataFrame({
        "a": [2.1, 4, None, 2.5],
        "b": [
            "j",
            "j",
            "l",  # Note: the third element has switched from the issue example, from `"k"` to `"l"`
            "l"
        ]
    })
    .with_columns(
        pl.col("a").rank().over("b")
        .name.suffix("_rank_over_b")
    )
)
print(df)

shape: (4, 3)
┌──────┬─────┬───────────────┐
│ a    ┆ b   ┆ a_rank_over_b │
│ ---  ┆ --- ┆ ---           │
│ f64  ┆ str ┆ f64           │
╞══════╪═════╪═══════════════╡
│ 2.1  ┆ j   ┆ 1.0           │
│ 4.0  ┆ j   ┆ 2.0           │
│ null ┆ l   ┆ null          │
│ 2.5  ┆ l   ┆ 1.0           │
└──────┴─────┴───────────────┘

Installed versions

--------Version info---------
Polars:               1.1.0
Index type:           UInt32
Platform:             macOS-14.5-arm64-arm-64bit
Python:               3.12.3 (main, Jun 12 2024, 23:52:12) [Clang 12.0.5 (clang-1205.0.22.11)]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            0.10.4
fsspec:               <not installed>
gevent:               <not installed>
great_tables:         <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         1.6.0
numpy:                2.0.0
openpyxl:             <not installed>
pandas:               <not installed>
pyarrow:              16.1.0
pydantic:             <not installed>
pyiceberg:            <not installed>
sqlalchemy:           <not installed>
torch:                <not installed>
xlsx2csv:             0.8.2
xlsxwriter:           <not installed>