Fill_null() changes Int64 data type to List(Int64) on aggregation

[lmocsi]

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.LazyFrame({'id':[1,1,2,2,3,3],'b':[11,None,33,44,None,66]})
print('ok:    ',df.group_by('id').agg(pl.col('b').sum()).collect_schema())
print('not ok:',df.group_by('id').agg(pl.col('b').sum().fill_null(0)).collect_schema())

Log output

ok:     Schema([('id', Int64), ('b', Int64)])
not ok: Schema([('id', Int64), ('b', List(Int64))])

Issue description

Upon aggregation, if I add .fill_null(0) to an Int64 column, then its datatype changes to List(Int64).

Expected behavior

Data type should not change with a .fill_null() addition.

Installed versions

--------Version info---------
Polars:              1.14.0
Index type:          UInt32
Platform:            Linux-5.10.223-212.873.amzn2.x86_64-x86_64-with-glibc2.26
Python:              3.9.20 | packaged by conda-forge | (main, Sep 30 2024, 17:49:10) 
[GCC 13.3.0]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
boto3                1.35.63
cloudpickle          3.1.0
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               2024.10.0
gevent               <not installed>
google.auth          2.36.0
great_tables         <not installed>
matplotlib           <not installed>
nest_asyncio         1.6.0
numpy                1.26.4
openpyxl             <not installed>
pandas               1.5.3
pyarrow              14.0.2
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>