[C++] String split kernels do not propagate nulls correctly on sliced input

[asfimport]

I am not sure if this is a specific test issue or valid behavior, but when writing a test in #8271 

The following test fails:

this->CheckUnary("split_pattern", R"(["foo bar", "foo", null])", list(this->type()),  //                  R"([["foo", "bar"], ["foo"], null])", &options);

with the following output

Failed:
Got: 
  [
    [
      [
        "foo",
        "bar"
      ]
    ],
    [
      [
        "foo"
      ],
      null
    ]
  ]
Expected: 
  [
    [
      [
        "foo",
        "bar"
      ]
    ],
    [
      [
        "foo"
      ],
      null
    ]
  ]

while the outputs are the same, the arrays are seen as unequal.

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

PRs and other links:

• GitHub Pull Request #8496

_{Note: This issue was originally created as ARROW-10208. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Just thinking out loud, but it may be that the types are different (perhaps list vs. large_list? perhaps one list has a non-nullable child field?).

[asfimport]

Antoine Pitrou / @pitrou:
Ok, so the actual problem is that the split kernel was reusing the input's null bitmap, even if the input is sliced.

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 8496
#8496