Format null arrays in Series like other series and format pl.List(pl.Null) arrays properly in dataframes.

[ghuls]

Problem description

Format null arrays in Series like other series and format pl.List(pl.Null) arrays properly in dataframes.

Currently:

• Formatting of dataframe columns with pl.List(pl.Null) does not work properly.
• A plain Polars Null series does not get printed like other series.

>>> tbl = pa.table({"a": [None, None], "b": [[None, None], [None, None]]})
>>> pl.from_arrow(tbl)
shape: (2, 2)
┌──────┬─────────────────┐
│ a    ┆ b               │
│ ---  ┆ ---             │
│ null ┆ list[null]      │
╞══════╪═════════════════╡
│ null ┆ fmt implemented │
│ null ┆ fmt implemented │
└──────┴─────────────────┘
>>> df = pl.from_arrow(tbl)
>>> df["a"]
nullarray
>>> pl.Series("a", [None, None], dtype=pl.Null)
nullarray

Compared with other (non pl.Null) Series:

>>> tbl_not_null = pa.table({"a": ["None", None], "b": [["None", None], [None, "None"]]})
>>> df = pl.from_arrow(tbl_not_null)
>>> df
shape: (2, 2)
┌──────┬────────────────┐
│ a    ┆ b              │
│ ---  ┆ ---            │
│ str  ┆ list[str]      │
╞══════╪════════════════╡
│ None ┆ ["None", null] │
│ null ┆ [null, "None"] │
└──────┴────────────────┘

>>> df["a"]
shape: (2,)
Series: 'a' [str]
[
        "None"
        null
]

>>> pl.Series("a", [None, "None"], dtype=pl.Utf8)
shape: (2,)
Series: 'a' [str]
[
        null
        "None"
]

[romanovacca]

I can try to to take this up. Just started learning rust, so I might need some help on this one :)

[romanovacca]

@ghuls can you assign this to me

[romanovacca]

Took a bit longer to pick this up. Will look at this issue this week @ghuls

[flepknor]

This issue has been adressed, no?
Sorry, I only just saw that it is solved only partly.