Streaming left join with multiple null categorical values raises PanicException

[daviskirk]

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

df1 = pl.LazyFrame({"_original_ix": pl.Series([0], dtype=pl.UInt32)})
df2 = pl.LazyFrame([
    pl.Series("_original_ix", [0, 1], dtype=pl.UInt32),
    pl.Series("l", [None, None], dtype=pl.Categorical(ordering='physical')),
])
df1.join(df2, on="_original_ix", how="left").collect(streaming=True) 

PanicException: assertion `left == right` failed: implementation error
  left: 1
 right: 2

Collecting without streaming works as expected.

shape: (1, 2)
┌──────────────┬──────┐
│ _original_ix ┆ l    │
│ ---          ┆ ---  │
│ u32          ┆ cat  │
╞══════════════╪══════╡
│ 0            ┆ null │
└──────────────┴──────┘

• modifying df1 to pl.LazyFrame({"_original_ix": [0]}) results in PanicException: not implemented: Null instead.

Log output

thread 'thread 'thread 'thread 'thread 'thread '<unnamed><unnamed><unnamed>thread '<unnamed><unnamed>' panicked at ' panicked at <unnamed>' panicked at ' panicked at ' panicked at ' panicked at <unnamed>crates/polars-ops/src/chunked_array/gather/chunked.rscrates/polars-ops/src/chunked_array/gather/chunked.rs:crates/polars-ops/src/chunked_array/gather/chunked.rscrates/polars-ops/src/chunked_array/gather/chunked.rs' panicked at :84crates/polars-ops/src/chunked_array/gather/chunked.rs:84thread ':5<unnamed>crates/polars-ops/src/chunked_array/gather/chunked.rs:
' panicked at :crates/polars-ops/src/chunked_array/gather/chunked.rsassertion `left == right` failed: implementation error
  left: 1
 right: 2:84:
:note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
:84845::crates/polars-ops/src/chunked_array/gather/chunked.rs8455::84:5::
assertion `left == right` failed: implementation error
  left: 1
 right: 25:

:
assertion `left == right` failed: implementation error
  left: 1
 right: 2assertion `left == right` failed: implementation error
  left: 1
 right: 2:


assertion `left == right` failed: implementation error
  left: 1
 right: 2
:
assertion `left == right` failed: implementation error
  left: 1
 right: 2584:5:
assertion `left == right` failed: implementation error
  left: 1
 right: 2

:
assertion `left == right` failed: implementation error
  left: 1
 right: 2

python3.11/site-packages/polars/lazyframe/frame.py:1934, in LazyFrame.collect(self, type_coercion, predicate_pushdown, projection_pushdown, simplify_expression, slice_pushdown, comm_subplan_elim, comm_subexpr_elim, no_optimization, streaming, background, _eager)
   1931 if background:
   1932     return InProcessQuery(ldf.collect_concurrently())
-> 1934 return wrap_df(ldf.collect())

PanicException: assertion `left == right` failed: implementation error
  left: 1
 right: 2

Issue description

Streaming left join with multiple null categorical values raises PanicException.

Expected behavior

The result should be the same as when not using streaming.

Installed versions

--------Version info---------
Polars:               0.20.14
Index type:           UInt32
Platform:             Linux-6.5.0-21-generic-x86_64-with-glibc2.38
Python:               3.11.8 (main, Feb 26 2024, 21:39:34) [GCC 11.2.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               2023.10.0
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
numpy:                1.26.4
openpyxl:             3.1.2
pandas:               2.2.1
pyarrow:              15.0.0
pydantic:             1.10.13
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           2.0.25
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>