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

[pitrou]

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

See this snippet:

>>> a = pa.array([11,11,11,42,42,5,None])
>>> r = pc.run_end_encode(a)

>>> pc.true_unless_null(a)
<pyarrow.lib.BooleanArray object at 0x7fa1fc54f5e0>
[
  true,
  true,
  true,
  true,
  true,
  true,
  null
]

>>> pc.true_unless_null(r)
<pyarrow.lib.BooleanArray object at 0x7fa1fc4cab60>
[
  true,
  true,
  true,
  true,
  true,
  true,
  true
]

[var-nan]

Hi @pitrou, I can give it a try. Can you assign this issue to me?

[pitrou]

@var-nan Looking forward to your PR!