align_frames regression: 1-row inputs with how="left" triggers panic

[rcliu623]

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

raw_dict = {"_grouping_id": [1], "a": [0], "b": [2]}
df = pl.DataFrame(raw_dict)
print(pl.align_frames(df, df, how="full", on="_grouping_id"))
print(pl.align_frames(df, df, how="left", on="_grouping_id"))

Log output

join parallel: true
join parallel: true
FULL join dataframes finished
FULL join dataframes finished
[shape: (1, 3)
┌──────────────┬─────┬─────┐
│ _grouping_id ┆ a   ┆ b   │
│ ---          ┆ --- ┆ --- │
│ i64          ┆ i64 ┆ i64 │
╞══════════════╪═════╪═════╡
│ 1            ┆ 0   ┆ 2   │
└──────────────┴─────┴─────┘, shape: (1, 3)
┌──────────────┬─────┬─────┐
│ _grouping_id ┆ a   ┆ b   │
│ ---          ┆ --- ┆ --- │
│ i64          ┆ i64 ┆ i64 │
╞══════════════╪═════╪═════╡
│ 1            ┆ 0   ┆ 2   │
└──────────────┴─────┴─────┘]
join parallel: true
left join: keys are sorted: use sorted merge join
join parallel: true
left join: keys are sorted: use sorted merge join
thread 'polars-0' panicked at crates/polars-ops/src/frame/join/dispatch_left_right.rs:174:10:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'polars-1' panicked at crates/polars-ops/src/frame/join/dispatch_left_right.rs:174:10:
called `Option::unwrap()` on a `None` value
Traceback (most recent call last):
  File "/home/polars_regression.py", line 7, in <module>
    print(pl.align_frames(df, df, how="left", on="_grouping_id"))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/.conda/lib/python3.11/site-packages/polars/functions/eager.py", line 475, in align_frames
    return F.collect_all(aligned_frames) if eager else aligned_frames  # type: ignore[return-value]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/.conda/lib/python3.11/site-packages/polars/functions/lazy.py", line 1714, in collect_all
    out = plr.collect_all(prepared)
          ^^^^^^^^^^^^^^^^^^^^^^^^^
pyo3_runtime.PanicException: called `Option::unwrap()` on a `None` value

Issue description

In 1.18, invoking align_frames with how="left" on 1-row dataframes now triggers a panic. Such error did not exist <=1.17.

Expected behavior

pl.align_frames(df, df, how="left", on="_grouping_id") should work as intended

Installed versions

--------Version info---------
Polars:              1.18.0
Index type:          UInt32
Platform:            Linux-4.18.0-477.36.1.x86_64-x86_64-with-glibc2.28
Python:              3.11.10 (main, Oct  3 2024, 07:29:13) [GCC 11.2.0]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
azure.identity       <not installed>
boto3                <not installed>
cloudpickle          3.1.0
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               2024.10.0
gevent               <not installed>
google.auth          <not installed>
great_tables         <not installed>
matplotlib           3.9.2
nest_asyncio         1.6.0
numpy                1.25.0
openpyxl             <not installed>
pandas               2.1.4
pyarrow              18.0.0
pydantic             2.9.2
pyiceberg            <not installed>
sqlalchemy           2.0.36
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>