Fix implementations of Iterator.next() for FindBugs IT_NO_SUCH_ELEMENT

[amcp]

• [ ] [INFO] com.amazon.janusgraph.diskstorage.dynamodb.iterator.ParallelScanner.next() can't throw NoSuchElementException [com.amazon.janusgraph.diskstorage.dynamodb.iterator.ParallelScanner] At ParallelScanner.java:[lines 110-116] IT_NO_SUCH_ELEMENT
• [ ] [INFO] com.amazon.janusgraph.diskstorage.dynamodb.iterator.ScanSegmentWorker.next() can't throw NoSuchElementException [com.amazon.janusgraph.diskstorage.dynamodb.iterator.ScanSegmentWorker] At ScanSegmentWorker.java:[lines 51-70] IT_NO_SUCH_ELEMENT
• [ ] [INFO] com.amazon.janusgraph.diskstorage.dynamodb.iterator.SequentialScanner.next() can't throw NoSuchElementException [com.amazon.janusgraph.diskstorage.dynamodb.iterator.SequentialScanner] At SequentialScanner.java:[lines 61-89] IT_NO_SUCH_ELEMENT