Github user greghogan commented on the issue:
https://github.com/apache/flink/pull/4525
`MutableObjectIterator#next` allows/requires a `null` result: "@return The
object or <code>null</code> if the iterator is exhausted.". The documentation
could be improved, e.g. in `MergeIterator` the returned object is not
necessarily the immediately passed argument.
`DataSourceTask` looks to be the only `InputFormat` consumer re-passing a
reuse object.
It seems the reason to allow returning `null` is handling, for example, a
bad record at the end of a file, such that `reachedEnd` would have returned
`false` but there is no record to return.
Object reuse is always optional so is not an issue for immutable types.
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