Github user StephanEwen commented on the issue:
https://github.com/apache/flink/pull/4525
I think `null` values are not really permitted at the moment, but I think
the InputFormats do not explicitly forbid them. That's why the logic is not
"run until returns null", but "run until reachedEnd()".
The change here implements a mixed contract like "run until reachedEnd() or
returns null", which is more complicated to document, enforce, test, and
understand for users.
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