str.json_decode panics on {} entry

[stinodego]

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

s = pl.Series(["{}"])
result = s.str.json_decode()
print(result)

Log output

thread '<unnamed>' panicked at crates/polars-arrow/src/array/struct_/mod.rs:117:52:
called `Result::unwrap()` on an `Err` value: ComputeError(ErrString("a StructArray must contain at least one field"))
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "/home/stijn/code/polars/py-polars/repro.py", line 4, in <module>
    result = s.str.json_decode()
             ^^^^^^^^^^^^^^^^^^^
  File "/home/stijn/code/polars/py-polars/polars/series/utils.py", line 107, in wrapper
    return s.to_frame().select_seq(f(*args, **kwargs)).to_series()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stijn/code/polars/py-polars/polars/dataframe/frame.py", line 7709, in select_seq
    return self.lazy().select_seq(*exprs, **named_exprs).collect(_eager=True)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stijn/code/polars/py-polars/polars/lazyframe/frame.py", line 1683, in collect
    return wrap_df(ldf.collect())
                   ^^^^^^^^^^^^^
pyo3_runtime.PanicException: called `Result::unwrap()` on an `Err` value: ComputeError(ErrString("a StructArray must contain at least one field"))

Issue description

Brought up in #9216

Empty structs do not exist, so this will error.

Expected behavior

This should raise a proper error.

Installed versions

main

[cmdlineluser]

It has popped up in read_json / read_ndjson also

• PanicError in read_ndjson and polars.str.json_decode with empty struct #13433
• read_json fails on empty objects #10234