Unexpected interaction between pl.when and pl.struct

[LucasBou]

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

import polars as pl

colint = pl.col("int")
colstr = pl.col("str")

a_struct = pl.struct(colint, colstr)
is_first_line = pl.col("id") == 1

resulting_expr = pl.when(is_first_line).then(a_struct)

data = pl.DataFrame({"id": [1, 2], "int": [5, 5], "str": ["str1", "str2"]})

result = data.select(resulting_expr)
print(data)
print(result)

Log output

No response

Issue description

When using a pl.Struct as the output of a when-then-otherwise expression and no otherwise clause is declared, we get a Struct with null fields.

Expected behavior

I would expect a null to be output instead of a struct with null fields, which would be consistent with the pl.when documentation :

If none of the conditions are True, an optional .otherwise() can be appended at the end. If not appended, and none of the conditions are True, None will be returned.

Installed versions

--------Version info---------
Polars:               0.20.21
Index type:           UInt32
Platform:             Windows-10-10.0.19045-SP0
Python:               3.12.2 (tags/v3.12.2:6abddd9, Feb  6 2024, 21:26:36) [MSC v.1937 64 bit (AMD64)]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               <not installed>
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         <not installed>
numpy:                1.26.4
openpyxl:             <not installed>
pandas:               2.2.1
pyarrow:              <not installed>
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[orlp]

This is a duplicate of #3462. Currently structs do not have outer nullability, and it would be a lot of work to add it.