Expr.slice no longer allows length = None

[cmdlineluser]

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

import polars as pl

df = pl.DataFrame({"foo": [1, 2, 3]})

df.select(pl.all().slice(1))
# InvalidOperationError: length must be integral for slice, not null

Log output

No response

Issue description

Previously, if no length was provided, all following rows would be selected.

If set to None, all rows starting at the offset will be selected.

Expected behavior

>>> df.select(pl.all().slice(1, pl.len()))
shape: (2, 1)
┌─────┐
│ foo │
│ --- │
│ i64 │
╞═════╡
│ 2   │
│ 3   │
└─────┘

Installed versions

--------Version info---------
Polars:               1.0.0
Index type:           UInt32
Platform:             macOS-13.6.1-arm64-arm-64bit
Python:               3.12.2 (main, Feb  6 2024, 20:19:44) [Clang 15.0.0 (clang-1500.1.0.2.5)]

----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>
great_tables:         <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         <not installed>
numpy:                1.26.4
openpyxl:             <not installed>
pandas:               2.2.1
pyarrow:              15.0.2
pydantic:             <not installed>
pyiceberg:            <not installed>
sqlalchemy:           <not installed>
torch:                <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[stinodego]

Thanks for the report!