RFR: 8297385: Remove duplicated null typos in javadoc

[Dongxu Wang]

8297385: Remove duplicated null typos in javadoc

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Commit messages:
 - 8297385: Remove duplicated null typo in javadoc

Changes: https://git.openjdk.org/jdk/pull/11311/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11311&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297385
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11311.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11311/head:pull/11311

PR: https://git.openjdk.org/jdk/pull/11311