str.concat should return fixed size data (not 0 or 1 length series)

[Julian-J-S]

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

# PROBLEM:
# Aggregation should not return different size Series but scalars (see problems and explanation below)
pl.Series(['a', 'b'], dtype=pl.Utf8).str.concat() # -> Series (1,) ["a-b"]
pl.Series([], dtype=pl.Utf8).str.concat() # -> Series (0,) []

# other aggregation (max)
pl.Series([1, 2], dtype=pl.UInt32).max() # -> 2
pl.Series([], dtype=pl.UInt32).max() # -> None

Log output

No response

Issue description

str.concat should behave like other vertical aggregation functions and allways return a single value.
Otherwise this leads to confusing / problematic behaviour like seen here: #12030

Example Problem from linked issue:

df = pl.DataFrame(
    {
        "id": ["1", "1", "2"],
        "text": ["a", "b", "c"],    # df1
        # "text": ["a", "b", None], # df2: None instead of "c"
    }
)

df.group_by("id").agg(
    list=pl.col("text").drop_nulls(),
    concat=pl.col("text").drop_nulls().str.concat(),
)

# df1: expected behaviour
┌─────┬────────────┬────────┐
│ id  ┆ list       ┆ concat │
│ --- ┆ ---        ┆ ---    │
│ str ┆ list[str]  ┆ str    │
╞═════╪════════════╪════════╡
│ 1   ┆ ["a", "b"] ┆ a-b    │
│ 2   ┆ ["c"]      ┆ c      │
└─────┴────────────┴────────┘

# df2: whhhyyy? =)
┌─────┬────────────┬───────────┐
│ id  ┆ list       ┆ concat    │
│ --- ┆ ---        ┆ ---       │
│ str ┆ list[str]  ┆ list[str] │ # >>>>> expect "str"
╞═════╪════════════╪═══════════╡
│ 1   ┆ ["a", "b"] ┆ ["a-b"]   │ # >>>>> expect "a-b" like above
│ 2   ┆ []         ┆ []        │ # >>>>> expect "" because `str.concat` on empyt list should be "" not Shape (0,) Series
└─────┴────────────┴───────────┘

Doing the same where the aggregation always produces a single value works fine:

pl.DataFrame(
    {
        "id": ["1", "1", "2"],
        "text": [10, 20, None],
    }
).group_by("id").agg(
    list=pl.col("text").drop_nulls(),
    concat=pl.col("text").drop_nulls().sum(),
)
┌─────┬───────────┬────────┐
│ id  ┆ list      ┆ concat │
│ --- ┆ ---       ┆ ---    │
│ str ┆ list[i64] ┆ i64    │
╞═════╪═══════════╪════════╡
│ 1   ┆ [10, 20]  ┆ 30     │
│ 2   ┆ []        ┆ 0      │ # <<<<< working as expected because [].sum = 0
└─────┴───────────┴────────┘

Expected behavior

str.concat should always return a single value

pl.Series(['a', 'b'], dtype=pl.Utf8).str.concat() # -> "a-b"
pl.Series([], dtype=pl.Utf8).str.concat() # -> "" (mimics join in rust & python on empty list)

Installed versions

--------Version info---------
Polars:              0.19.11
Index type:          UInt32
Platform:            Windows-10-10.0.19045-SP0
Python:              3.11.6 (tags/v3.11.6:8b6ee5b, Oct  2 2023, 14:57:12) [MSC v.1935 64 bit (AMD64)]```

</details>

[reswqa]

Closed as complete in #12066.