DataFrame.unique() should raise if any subset column doesn't exist on empty frame.

[mrjsj]

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(
    {
        "ID": [],
        "Name": []
    },
    schema={"ID": pl.Int64, "Name": pl.String}
)
df = df.unique(subset="id")
print(df)
# shape: (0, 2)
# ┌──────┬──────┐
# │ ID   ┆ Name │
# │ ---  ┆ ---  │
# │ null ┆ null │
# ╞══════╪══════╡
# └──────┴──────┘

df = pl.DataFrame(
    {
        "ID": [1, 2, 1, 2],
        "Name": ["foo", "bar", "baz", "baa"]
    },
    schema={"ID": pl.Int64, "Name": pl.String}
)
df = df.unique(subset="id")
# raises `polars.exceptions.ColumnNotFoundError: "id" not found`

Log output

No response

Issue description

Using .unique() on an empty dataframe with a subset column which doesn't exist in the dataframe doesn't raise an error.

This is inconsistent with other methods, i.e. .select()

Expected behavior

.unique() should raise an error if any subset column is not in the dataframe.

Installed versions

--------Version info---------
Polars:              1.16.0
Index type:          UInt32
Platform:            macOS-15.0.1-arm64-arm-64bit
Python:              3.12.6 (main, Sep  6 2024, 19:03:47) [Clang 15.0.0 (clang-1500.3.9.4)]
LTS CPU:             True

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
boto3                <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            0.21.0
fastexcel            <not installed>
fsspec               2024.10.0
gevent               <not installed>
google.auth          <not installed>
great_tables         <not installed>
matplotlib           <not installed>
nest_asyncio         <not installed>
numpy                2.1.3
openpyxl             <not installed>
pandas               2.2.3
pyarrow              18.1.0
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>

[nnlnr]

👋 Hi there - I'd like to pick this up if it's still available.