`read_ndjson` PanicException with null input

[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.read_ndjson(b"null")

Log output

thread '<unnamed>' panicked at crates/polars-arrow/src/array/struct_/mod.rs:258:37:
called `Result::unwrap()` on an `Err` value: ComputeError(ErrString("Struct array must be created with a DataType whose physical type is Struct"))

Issue description

read_json raises a ComputeError.

pl.read_json(b"null")
# ComputeError: can only deserialize json objects

Expected behavior

No panic.

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

[deanm0000]

orjson returns None from orjson.loads(b"null") so should we return a (0,0) df instead of raising? I don't have a strong opinion, raising makes sense too since a 0,0 df is arguably not the same as None.

Semi-related, should pl.DataFrame().item() return None instead of raising?