RFR: 8343622: AesDkCrypto.stringToKey should not return null

[Weijun Wang]

When an exception happens inside these stringToKey() methods, it returns a null. This would lead to unexpected NPE some time in the future.

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Commit messages:
 - 8343622: AesDkCrypto.stringToKey should not return null

Changes: https://git.openjdk.org/jdk/pull/21908/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21908&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343622
  Stats: 62 lines in 5 files changed: 55 ins; 6 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21908.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21908/head:pull/21908

PR: https://git.openjdk.org/jdk/pull/21908