[C++] Unary kernels that results in a list have no preallocated offset buffer

[asfimport]

I noticed in

#8271

That a string->list[string] kernel does not have the offsets preallocated in the output. I believe there is a preference for not doing allocations in kernels, so this can be optimized at a higher level. I think it can also be done in this case. 

Reporter: Maarten Breddels / @maartenbreddels
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #8498

_{Note: This issue was originally created as ARROW-10207. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
@wesm What do you think?

[asfimport]

Wes McKinney / @wesm:
It seems OK for there to be the option to preallocate the offsets and validity buffer, so long as it's straightforward to turn it off when the implementation will be using a builder

[asfimport]

Antoine Pitrou / @pitrou:
Hmm, this report may be mistaken. A scalar kernel always computes the output nulls by default, and there is no exception for list-returning scalar kernels.

[asfimport]

Antoine Pitrou / @pitrou:
Oops, the report is about the list offsets, nevermind.

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 8498
#8498