fill_null changes dtype

[Vincenthays]

Checks

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

Reproducible example

(
    pl.LazyFrame(
        pl.Series('a', [1, 2, None], dtype=pl.UInt16)
    )
    .inspect()
    .with_columns(
        pl.col('a').fill_null(0)
    )
    .collect()
)

Log output

shape: (3, 1)
┌──────┐
│ a    │
│ ---  │
│ u16  │
╞══════╡
│ 1    │
│ 2    │
│ null │
└──────┘
shape: (3, 1)
┌─────┐
│ a   │
│ --- │
│ i32 │
╞═════╡
│ 1   │
│ 2   │
│ 0   │
└─────┘

Issue description

fill_null should not change dtype

pl.Series('a', [1, 2, None], dtype=pl.UInt32).fill_null(0).dtype != pl.UInt32

Expected behavior

Don't change dtype

Installed versions

Polars:               0.20.23
Index type:           UInt32
Platform:             Linux-6.6.22-linuxkit-aarch64-with-glibc2.36
Python:               3.12.3 (main, Apr 24 2024, 07:13:43) [GCC 12.2.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               <not installed>
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         <not installed>
numpy:                <not installed>
openpyxl:             <not installed>
pandas:               <not installed>
pyarrow:              <not installed>
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>```

</details>

[cmdlineluser]

For reference: https://github.com/pola-rs/polars/issues/13789#issuecomment-1937525648

We should actually raise, but we currently do not have the infrastructure to do so yet.

[stinodego]

Closing as duplicate of #13789