RFR: Skipping null strings during popping dedup string candidates

[Zhengyu Gu]

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/


Test:
   tier3_gc_shenandoah with -XX:+UseStringDeduplication

Thanks,

-Zhengyu