fix: Fix all-null list aggregations returning Null dtype

[lukemanley]

closes #17361

import polars as pl

df = pl.DataFrame(
    {
        "a": [None, None],
        "b": [None, None],
    },
    schema={
        "a": pl.List(pl.String),
        "b": pl.List(pl.Duration),
    }
)

df.select(
    pl.col("a").list.min(),
    pl.col("b").list.sum(),
)

BEFORE:

shape: (2, 2)
┌──────┬──────┐
│ a    ┆ b    │
│ ---  ┆ ---  │
│ null ┆ null │
╞══════╪══════╡
│ null ┆ null │
│ null ┆ null │
└──────┴──────┘

AFTER:

shape: (2, 2)
┌──────┬──────────────┐
│ a    ┆ b            │
│ ---  ┆ ---          │
│ str  ┆ duration[μs] │
╞══════╪══════════════╡
│ null ┆ null         │
│ null ┆ null         │
└──────┴──────────────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.20%. Comparing base (ea1ea5a) to head (f138f61).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20992      +/-   ##
==========================================
- Coverage   79.20%   79.20%   -0.01%     
==========================================
  Files        1583     1583              
  Lines      225109   225112       +3     
  Branches     2581     2581              
==========================================
- Hits       178303   178301       -2     
- Misses      46216    46221       +5     
  Partials      590      590              

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