fix: nested null data type Series treated as an empty Series

[eitsupi]

A follow up for #1021

This change will allow for more proper conversion from lists that contain NULLs or empty lists.

For example, in 0.15.1, a strange error occurs in the following case:

> as_polars_series(list(list(), list(1, 2)))
Error: Execution halted with the following contexts
   0: In R: in pl$Series():
   0: During function call [as_polars_series(list(list(), list(1, 2)))]
   1: Encountered the following error in Rust-Polars:
        data types don't match: When building series from R list; some parsed sub-elements did not match: One element was f64 and another was list[f64]

[eitsupi]

Updating the NEWS file will cause conflicts with #1022, so I want to update later.

[etiennebacher]

LGTM, thanks