RFR: 6205692: (spec) javax.crypto.MacSpi.engineUpdate(ByteBuffer input): NPE should be specified

[Kevin Driver]

add @exception message to indicate an NPE might be thrown when the input parameter is null

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Commit messages:
 - https://bugs.openjdk.org/browse/JDK-6205692 : add @exception message to indicate an NPE might be thrown when the input parameter is null

Changes: https://git.openjdk.org/jdk/pull/9859/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9859&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6205692
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9859.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9859/head:pull/9859

PR: https://git.openjdk.org/jdk/pull/9859