Inconsistent dtypes after .implode in group_by depending on the emptyness of the frame

[NicolasMuellerQC]

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(data=dict(x = ["a", "b"], g=[1,1]))

df.group_by("g").agg( pl.col("x").implode().list.first())
# Yields
# shape: (1, 2)
#┌─────┬───────────┐
#│ g   ┆ x         │
#│ --- ┆ ---       │
#│ i64 ┆ list[str] │
#╞═════╪═══════════╡
#│ 1   ┆ ["a"]     │
#└─────┴───────────┘
# as expected


df.head(0).group_by("g").agg( pl.col("x").implode().list.first())
# Yields
# shape: (0, 2)
#┌─────┬─────┐
#│ g   ┆ x   │
#│ --- ┆ --- │
#│ i64 ┆ str │
#╞═════╪═════╡
#└─────┴─────┘
# Note the different dtype!



## Example where this is relevant:
df.group_by("g").agg(pl.col("x").filter(pl.col("x") == pl.col("x").filter(pl.col("x").is_last_distinct()).implode().list.get(-1)))
# Yields 
#shape: (1, 2)
#┌─────┬───────────┐
#│ g   ┆ x         │
#│ --- ┆ ---       │
#│ i64 ┆ list[str] │
#╞═════╪═══════════╡
#│ 1   ┆ ["b"]     │
#└─────┴───────────┘
# while
df.head(0).group_by("g").agg(pl.col("x").filter(pl.col("x") == pl.col("x").filter(pl.col("x").is_last_distinct()).implode().list.get(-1)))
# raises
# pyo3_runtime.PanicException: called `Result::unwrap()` on an `Err` value: SchemaMismatch(ErrString("invalid series dtype: expected `List`, got `str`"))

Log output

No response

Issue description

implode().list.first() and other elements of the .list namespace return inconsistent dtypes in a group context depending on whether the frame is empty or not.

Expected behavior

I expect the dtypes in the empty case to be the same as in the non-empty case. In particular

df.group_by("g").agg(pl.col("x").filter(pl.col("x") == pl.col("x").filter(pl.col("x").is_last_distinct()).implode().list.get(-1)))

should not raise but return a frame with 0 rows with schema g: i64, x: list[str].

Installed versions

--------Version info---------
Polars:              1.9.0
Index type:          UInt32
Platform:            macOS-14.6.1-arm64-arm-64bit
Python:              3.12.7 | packaged by conda-forge | (main, Oct  4 2024, 15:57:01) [Clang 17.0.6 ]
----Optional dependencies----
adbc_driver_manager  <not installed>
altair               4.2.2
cloudpickle          3.0.0
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               2024.9.0
gevent               <not installed>
great_tables         <not installed>
matplotlib           3.9.2
nest_asyncio         1.6.0
numpy                1.26.4
openpyxl             3.1.5
pandas               2.2.3
pyarrow              17.0.0
pydantic             2.9.2
pyiceberg            <not installed>
sqlalchemy           2.0.35
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           3.2.0

[cmdlineluser]

This is also fixed by:

• fix: Fix invalid list collection in expression engine #19191

(df.head(0)
   .group_by("g")
   .agg(
       pl.col("x").filter(pl.col("x") == pl.col("x").filter(pl.col("x").is_last_distinct()).implode().list.get(-1))
    )
)

# shape: (0, 2)
# ┌─────┬───────────┐
# │ g   ┆ x         │
# │ --- ┆ ---       │
# │ i64 ┆ list[str] │
# ╞═════╪═══════════╡
# └─────┴───────────┘

[ritchie46]

Nice!