Docker environment: check for None on stdout/stderr response

[stsewd]

Docker returns None if there wasn't anything printed
to stdout/stderr.

[agjohnson]

Alternatively, you could set defaults at the top -- plus a walrus!

Suggested change

	cmd_stdout, cmd_stderr = out
	cmd_stdout, cmd_stderr = (out := ("", ""))

If it helps to default cmd_stdout to "" instead of None.

[stsewd]

This overrides the real output

>>> out = (None, "foo")
>>> cmd_stdout, cmd_stderr = (out := ("", ""))
>>> cmd_stdout
''
>>> cmd_stderr
''

[humitos]

I think the problem should be fixed in

readthedocs.org/readthedocs/doc_builder/environments.py

Line 197 in c3ace24

sanitized = None

instead of where it's fixed in this PR.

If output=None or invalid, it returns None. So, in both cases, we want to return "" is seems. I think it's easier to revert this commit and apply this solution which will also remove @ericholscher's concern from #9238