Java 8 RFR 8024331: j.u.Map.computeIfPresent() default/nondefault implementations don't throw NPE if the remappingFunction is null and the key is absent

[Brian Burkhalter]

Please review this proposed patch at your convenience.

Summary:	Explicitly check for null remappingFunction parameter (and clean up similar code for consistency).
Issue:		http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8024331
Webrev:		http://cr.openjdk.java.net/~bpb/8024331/

Thanks,

Brian