[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++