When length = None in arr.slice we get a ComputeError

[braaannigan]

Polars version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of Polars.

Issue description

According to the API the following should index from the first value to the end of the array, but instead raises a ComputeError exception

Reproducible example

import polars as pl
vals = [[1, 2, 3, 4], [10, 2, 1]]
s = pl.Series("a", vals)
s.arr.slice(1)
---------------------------------------------------------------------------
ComputeError                              Traceback (most recent call last)
Cell In[13], line 3
      1 vals = [[1, 2, 3, 4], [10, 2, 1]]
      2 s = pl.Series("a", vals)
----> 3 s.arr.slice(1)

File /usr/local/lib/python3.10/site-packages/polars/internals/series/utils.py:98, in call_expr.<locals>.wrapper(self, *args, **kwargs)
     96     expr = getattr(expr, namespace)
     97 f = getattr(expr, func.__name__)
---> 98 return s.to_frame().select(f(*args, **kwargs)).to_series()

File /usr/local/lib/python3.10/site-packages/polars/internals/dataframe/frame.py:5620, in DataFrame.select(self, exprs)
   5524 def select(
   5525     self: DF,
   5526     exprs: str
   (...)
   5529     | Sequence[str | pli.Expr | pli.Series | pli.WhenThen | pli.WhenThenThen],
   5530 ) -> DF:
   5531     """
   5532     Select columns from this DataFrame.
   5533 
   (...)
   5617 
   5618     """
   5619     return self._from_pydf(
-> 5620         self.lazy().select(exprs).collect(no_optimization=True)._df
   5621     )

File /usr/local/lib/python3.10/site-packages/polars/utils.py:329, in deprecated_alias.<locals>.deco.<locals>.wrapper(*args, **kwargs)
    326 @functools.wraps(fn)
    327 def wrapper(*args: P.args, **kwargs: P.kwargs) -> T:
    328     _rename_kwargs(fn.__name__, kwargs, aliases)
--> 329     return fn(*args, **kwargs)

File /usr/local/lib/python3.10/site-packages/polars/internals/lazyframe/frame.py:1168, in LazyFrame.collect(self, type_coercion, predicate_pushdown, projection_pushdown, simplify_expression, no_optimization, slice_pushdown, common_subplan_elimination, streaming)
   1157     common_subplan_elimination = False
   1159 ldf = self._ldf.optimization_toggle(
   1160     type_coercion,
   1161     predicate_pushdown,
   (...)
   1166     streaming,
   1167 )
-> 1168 return pli.wrap_df(ldf.collect())

ComputeError: could not extract number from AnyValue of dtype: 'Unknown'

Expected behavior

Slices to end of the array

Installed versions

---Version info---
Polars: 0.15.7
Index type: UInt32
Platform: Linux-5.10.104-linuxkit-x86_64-with-glibc2.28
Python: 3.10.1 (main, Dec 21 2021, 09:50:13) [GCC 8.3.0]
---Optional dependencies---
pyarrow: 10.0.1
pandas: 1.5.2
numpy: 1.23.5
fsspec: <not installed>
connectorx: 0.3.1
xlsx2csv: 0.8
matplotlib: 3.6.2

[jr200]

I also see this issue on 0.14.31. This workaround appears to fix the issue for now....

import polars as pl
vals = [[1, 2, 3, 4], [10, 2, 1]]
s = pl.Series("a", vals)
s.arr.slice(1, s.arr.lengths())