SI-8475 GroupedIterator is also lazy when padded

This is the related case which dnlgtm in the previous fix.

The old code seems to be tilting for laziness, but we have
to fill the buffer with the current group anyway, so there
is no reason to be coy about calling ArrayBuffer.length.
Merge pull request #3795 from som-snytt/issue/8475

SI-8475 GroupedIterator is also lazy when padded