feat(rust): Format null arrays in Series

[romanovacca]

Resolves #7153

null array now is shown as:

shape: (2,)
Series: 'a' [nullarray]
[
        null
        null
] 

instead of

nullarray

Could refactor the code a bit cause it overlaps with format_array a lot, so there is quite some repitition, but made this PR to see if this is in the right direction at all.

[romanovacca]

CI failures seem unrelated?

[orlp]

Can you rebase based on main? There's a lot of stuff in your PR that shouldn't be in there.

[romanovacca]

Can you rebase based on main? There's a lot of stuff in your PR that shouldn't be in there.

Yeah i was fighting with that, never used rebase before. Should be good now!

[romanovacca]

>>> df_null 
shape: (2, 1)
┌──────┐
│ a    │
│ ---  │
│ null │
╞══════╡
│ null │
│ null │
└──────┘


>>> df_null["a"]
shape: (2,)
Series: 'a' [null]
[
        null
        null
]
>>> 

[ritchie46]

Thank you @romanovacca. Looks better indeed. :)