RFR: 8025067: (xs) Arrays.parallelPrefix may fail to throw NPE for null op

[Mike Duigou]

Hello all;

This changeset fixes a small corner case where Arrays.parallelPrefix (all 
variants) failed to throw NPE if the operation was null. If the array or range 
was empty then op was never examined. The solution in this patch is to add an 
an explicit Objects.requireNonNull(op) check.

http://cr.openjdk.java.net/~mduigou/JDK-8025067/0/webrev/

Cheers,

Mike