RFR: Skipping null strings during popping dedup string candidates

[Aleksey Shipilev]

On 07/17/2018 04:39 AM, Zhengyu Gu wrote:
> When a NULL string is popped from StringDedupQueue, it means that there is not more candidate for
> StringDedupThread to process and  put it into wait state.
> 
> However, when queues are purged during GC, they may contain NULL string entries, so we should skip
> these entries during popping string candidates from queue, only return NULL when there is no more
> entries in queues.
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/strdedup_pop_empty/webrev.00/

OK.

-Aleksey