RFR: 8295919: java.security.MessageDigest.isEqual does not adhere to @implNote

[Kevin Driver]

Fix JDK-8295919 by updating the javadoc to explain that a null or zero-length `digestb` will also result in a short-circuit response

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Commit messages:
 - update copyright year
 - Fix JDK-8295919 by updating the javadoc to explain that a null or zero-length  will also result in a short-circuit response

Changes: https://git.openjdk.org/jdk/pull/15933/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15933&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295919
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15933.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15933/head:pull/15933

PR: https://git.openjdk.org/jdk/pull/15933