fix(rust, python): deal with empty structs

[ritchie46]

closes #6001

[pebble2050]

@ritchie46
This bug not fixed yet. When have have two items, got error:
Could not create a new DataFrame from Series. The Series have different lengths. Got [shape: (2,)

import polars as pl
from dataclasses import dataclass

@dataclass
class TestClass:
    int_field: int
    dict_field: dict

t = TestClass(int_field=10, dict_field={})

df = pl.DataFrame([t,t])

[mikeam565]

@ritchie46 Hello. This bug has not been totally fixed. If a field is empty in all json entries, then polars will still throw OutOfSpec("A StructArray must contain at least one field"). Eg.
[ {"a":"A", "test":"Y"}, {"a":"A", "test":"N", "anObject":{}}, {"a":"A", "test":"N", "anObject":{}} ]
The above json if parsed with JsonReader will throw an error.

[avimallu]

You might need to raise another bug report. This is a very old PR, might get missed.