Fix readline_channel crash and None return on default timeout

[archy-rock3t-cloud]

What type of PR is this?

/kind bug

What this PR does / why we need it:

With the default timeout, readline_channel converted None to float("inf") and crashed in update() when it reached select.poll().poll(). An infinite timeout now maps to a blocking poll, and an expired finite timeout returns '' or b'' like the other read methods instead of None.

Which issue(s) this PR fixes:

Fixes #2621

Does this PR introduce a user-facing change?

Fix readline_channel, readline_stdout and readline_stderr crashing with OverflowError when using the default timeout, and returning None when a timeout expires.

[kubernetes-prow]

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This pull-request has been approved by: archy-rock3t-cloud
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