fix: accidental panic if predicate selects no files

[ritchie46]

fixes #12570

[ddutt]

FWIW, I tried the build in panic branch and i still see an error. Its different from before:

>>> lf = pl.scan_parquet('/home/ddutt/work/suzieq/tests/data/parquet/inventory/**/*.parquet').filter(pl.col.namespace=="foo").select(['namespace', 'hostname', 'timestamp']).collect()
thread '<unnamed>' panicked at /home/ddutt/work/polars/crates/polars-core/src/utils/mod.rs:576:34:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ddutt/work/polars/py-polars/polars/utils/deprecation.py", line 100, in wrapper
    return function(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ddutt/work/polars/py-polars/polars/lazyframe/frame.py", line 1788, in collect
    return wrap_df(ldf.collect())
                   ^^^^^^^^^^^^^
pyo3_runtime.PanicException: called `Option::unwrap()` on a `None` value