RFR: 8281626: NonblockingQueue should use nullptr

[Kim Barrett]

Please review this change to use nullptr instead of NULL throughout the NonblockingQueue class.

Testing:
mach5 tier1

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Commit messages:
 - use nullptr throughout

Changes: https://git.openjdk.java.net/jdk/pull/7438/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7438&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281626
  Stats: 39 lines in 2 files changed: 0 ins; 0 del; 39 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7438.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7438/head:pull/7438

PR: https://git.openjdk.java.net/jdk/pull/7438