Panic when building a Series of lists without an inner dtype

[petrosbar]

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• 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

pl.Series("", [["a"], []], dtype=pl.List)
# PanicException: called `Result::unwrap()` on an `Err` value: SchemaMismatch(ErrString("invalid series dtype: expected `String`, got `null`"))

Issue description

Building a Series of lists without specifying an inner dtype, panics, when values contain an empty list. (or a list with only Nones.)

Expected behavior

pl.Series("", [["a"], []], dtype=pl.List)

# shape: (2,)
# Series: '' [list[str]]
# [
#         ["a"]
#         []
# ]