fix: Do not raise when constructing from a list of Series with Nones

[petrosbar]

Currently, constructing a Series from a list of Series which also contains Nones raises:

import polars as pl

inner = pl.Series("inner", [1, 2, 3])
pl.Series("a", [inner, None])
# AttributeError: 'NoneType' object has no attribute '_s'

This PR fixes this:

import polars as pl

inner = pl.Series("inner", [1, 2, 3])
pl.Series("a", [inner, None])

Output:

Series: 'a' [list[i64]]
[
        [1, 2, 3]
        null
]

[ritchie46]

Good fix. Thanks a lot.