Panick since first release candidate when expressions in select() return a different number of rows

[etiennebacher]

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 os
os.environ["POLARS_VERBOSE"] = "1"
import polars as pl

df = pl.DataFrame({
  "A": [1, 2],
  "B": [3, 4],
  "grp": ["A", "B"]
})

df.select(
  pl.col("A").sum(),
  pl.col("B").sum().over("grp")
)

Log output

thread '<unnamed>' panicked at crates/polars-mem-engine/src/executors/projection_utils.rs:120:38:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/etienne/.local/lib/python3.10/site-packages/polars/dataframe/frame.py", line 8499, in select
    return self.lazy().select(*exprs, **named_exprs).collect(_eager=True)
  File "/home/etienne/.local/lib/python3.10/site-packages/polars/lazyframe/frame.py", line 1909, in collect
    return wrap_df(ldf.collect(callback))
pyo3_runtime.PanicException: called `Option::unwrap()` on a `None` value

Issue description

This worked in 0.20.31.

Expected behavior

shape: (2, 2)
┌─────┬─────┐
│ A   ┆ B   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 3   ┆ 3   │
│ 3   ┆ 4   │
└─────┴─────┘

Installed versions

--------Version info---------
Polars:               1.0.0-rc.2
Index type:           UInt32
Platform:             Linux-6.5.0-41-generic-x86_64-with-glibc2.35
Python:               3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0]

----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:                1.21.5
openpyxl:             <not installed>
pandas:               <not installed>
pyarrow:              <not installed>
pydantic:             <not installed>
pyiceberg:            <not installed>
sqlalchemy:           <not installed>
torch:                <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[MarcoGorelli]

thanks @etiennebacher for the report

from git bisect, this is due to #16688 - going to ping @coastalwhite then (no blame of course! 🤗 )

[etiennebacher]

Worth noting that I noticed this because the README example fails, so it's related to #17309

[ritchie46]

I will take this one.