Can't read ndjson file with null object

[pebble2050]

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Issue description

Can't read ndjson file with null object. Simplar to #6075 But need another solve approach, Since json file may have too many fileds, passing type as paramter may be not accepable.

Reproducible example

import polars as pl
from io import BytesIO

nj = "{\"a\":1,\"b\":{}}\n{\"a\":2,\"b\":{}}\n"

buf = BytesIO(bytes(nj,'ascii'))

df = pl.read_ndjson(buf)
print(df)

Expected behavior

read normally

Installed versions

---Version info---
Polars: 0.15.14
Index type: UInt32
Platform: Windows-10-10.0.19045-SP0
Python: 3.9.7 (default, Sep 16 2021, 16:59:28) [MSC v.1916 64 bit (AMD64)]
---Optional dependencies---
pyarrow: 10.0.1
pandas: 1.5.2
numpy: 1.22.3
fsspec: 2022.11.0
connectorx: <not installed>
xlsx2csv: <not installed>
matplotlib: 3.5.2

[ritchie46]

@universalmind303 could you take a look at this one?