RFR: 8358316: PKCS8Key.getEncoded() can throw NPE after JDK-8298420

[Aleksey Shipilev]

Found by static analysis, see more details in the bug.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `jdk_security`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/25581/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25581&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358316
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25581.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25581/head:pull/25581

PR: https://git.openjdk.org/jdk/pull/25581