[C++] `hash_count` kernel miscounts when run-end encoded array contains null

[felipecrv]

Describe the bug, including details regarding any error messages, version, and platform.

Passing run-end encoded array in unit tests leads to these failures:

[ RUN      ] GroupBy/GroupBy.CountOnly/0
/home/felipeo/code/arrow/cpp/src/arrow/testing/gtest_util.cc:100: Failure
Failed

@@ -0, +0 @@
-{key_0: 1, hash_count: 2}
+{key_0: 1, hash_count: 3}
@@ -2, +2 @@
-{key_0: 3, hash_count: 0}
+{key_0: 3, hash_count: 2}
Expected:
  -- is_valid: all not null
  -- child 0 type: int64
    [
      1,
      2,
      3,
      null
    ]
  -- child 1 type: int64
    [
      2,
      3,
      0,
      2
    ]
Actual:
  -- is_valid: all not null
  -- child 0 type: int64
    [
      1,
      2,
      3,
      null
    ]
  -- child 1 type: int64
    [
      3,
      3,
      2,
      2
    ]
    ```

### Component(s)

C++