Review Request: GIRAPH-124: Combiner should return Iterable instead of M or null.

[Claudio Martella]

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https://reviews.apache.org/r/3592/
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Review request for giraph.


Summary
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Fixed the null check on the returned value and the javadoc..


Diffs
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Diff: https://reviews.apache.org/r/3592/diff


Testing
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local and MR unit tests


Thanks,

Claudio