[C++][Compute] `true_unless_null` kernel output incorrect when run-end encoded array contains null

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

See this snippet:
```python
>>> a = pa.array([11,11,11,42,42,5,None])
>>> r = pc.run_end_encode(a)

>>> pc.true_unless_null(a)

[
true,
true,
true,
true,
true,
true,
null
]

>>> pc.true_unless_null(r)

[
true,
true,
true,
true,
true,
true,
true
]
```