ShuffleBlockFetcherIterator should fail if it receives zero-size blocks

In the shuffle layer, we guarantee that zero-size blocks will never be requested (a block containing zero records is always 0 bytes in size and is marked as empty such that it will never be legitimately requested by executors). However, we failed to take advantage of this in the shuffle-read path: the existing code did not explicitly check whether blocks are non-zero-size.

We should add `buf.size != 0` checks to ShuffleBlockFetcherIterator to take advantage of this invariant and prevent potential data loss / corruption issues.