DataFrame.pivot returns incorrect results when multi-index contains a struct

[MarcoGorelli]

Checks

• 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

df = pl.DataFrame(
    {
        "a": [1, 2, 1],
        "b": [{"a": 1}, {"a": 1}, {"a": 2}],
        "c": ["x", "y", "y"],
        "d": [1, 1, 3],
    }
)
print(df.pivot(index=("b", "d"), values="a", columns="c"))

Log output

shape: (3, 4)
┌───────────┬─────┬──────┬──────┐
│ b         ┆ d   ┆ x    ┆ y    │
│ ---       ┆ --- ┆ ---  ┆ ---  │
│ struct[1] ┆ i64 ┆ i64  ┆ i64  │
╞═══════════╪═════╪══════╪══════╡
│ {null}    ┆ 1   ┆ 1    ┆ null │
│ {null}    ┆ 1   ┆ null ┆ 2    │
│ {null}    ┆ 3   ┆ null ┆ 1    │
└───────────┴─────┴──────┴──────┘

Issue description

Similar to #14101, but it goes down a different branch so I think it deserves a separate issue to make sure that both parts get addressed

Expected behavior

I think it should be

shape: (2, 4)
┌──────────┬─────┬──────┬─────┐
│ b        ┆ d   ┆ x    ┆ y   │
│ ---      ┆ --- ┆ ---  ┆ --- │
│ struct[1]┆ i64 ┆ i64  ┆ i64 │
╞══════════╪═════╪══════╪═════╡
│ {1}      ┆ 1   ┆ 1    ┆ 2   │
│ {2}      ┆ 3   ┆ null ┆ 1   │
└──────────┴─────┴──────┴─────┘

Installed versions

--------Version info---------
Polars:               0.20.7
Index type:           UInt32
Platform:             Linux-5.15.133.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
Python:               3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          3.0.0
connectorx:           <not installed>
deltalake:            <not installed>
fsspec:               2023.12.2
gevent:               <not installed>
hvplot:               0.9.1
matplotlib:           3.8.2
numpy:                1.26.3
openpyxl:             <not installed>
pandas:               2.2.0
pyarrow:              15.0.0
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           2.0.25
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>```

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