procutil: use os.devnull if sys.stdout/err is None

According to Yuya, it's better to make `procutil.stdin/err/out` always
available.

This is a bandaid until someone suggests something better.
7 jobs for topic/default/chg-test in 34 minutes and 19 seconds (queued for 7 seconds)
latest
Status Job ID Name Coverage
  Tests
passed #137941
checks-py2

00:04:51

passed #137942
test-py2-chg

00:21:57

passed #137943
test-py2-pure-chg

00:23:00

passed #137944
test-py2-rust-chg

00:25:16

passed #137945
test-py3-chg

00:10:05

passed #137946
test-py3-pure-chg

00:09:39

passed #137947
test-py3-rust-chg

00:09:59