concat of Struct ignoring null fields

[cmdlineluser]

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

pl.concat(
    [
        pl.DataFrame([[{"x": "foo", "y": "bar"}]]),
        pl.DataFrame([[{"z": None}]]),
    ],
)    

# shape: (2, 1)
# ┌───────────────┐
# │ column_0      │
# │ ---           │
# │ struct[2]     │
# ╞═══════════════╡
# │ {"foo","bar"} │
# │ {null,null}   │
# └───────────────┘

Log output

␤

Issue description

If a non-null value is used, it errors.

pl.concat(
    [
        pl.DataFrame([[{"x": "foo", "y": "bar"}]]),
        pl.DataFrame([[{"z": "baz"}]]),
    ],
)    
# SchemaError: cannot append series, data types don't match

Expected behavior

SchemaError

Installed versions

--------Version info---------
Polars:              1.29.0
Index type:          UInt32
Platform:            macOS-13.6.1-arm64-arm-64bit-Mach-O
Python:              3.13.0 (main, Oct  7 2024, 05:02:14) [Clang 15.0.0 (clang-1500.1.0.2.5)]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
boto3                <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            <not installed>
fastexcel            0.12.0
fsspec               <not installed>
gevent               <not installed>
google.auth          <not installed>
great_tables         0.14.0
matplotlib           <not installed>
nest_asyncio         <not installed>
numpy                2.1.3
openpyxl             3.1.5
pandas               2.2.3
pyarrow              18.0.0
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           3.2.0