RFR: 8289274: Cleanup unnecessary null comparison before instanceof check in security modules

[Andrey Turbanov]

Update code checks both non-null and instance of a class in security classes.
The checks and explicit casts could also be replaced with pattern matching for the instanceof operator.

See similar cleanup in java.base - [JDK-8258422](https://bugs.openjdk.java.net/browse/JDK-8258422)

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Commit messages:
 - [PATCH] Cleanup unnecessary null comparison before instanceof check in security modules

Changes: https://git.openjdk.org/jdk/pull/9284/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9284&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289274
  Stats: 94 lines in 14 files changed: 0 ins; 40 del; 54 mod
  Patch: https://git.openjdk.org/jdk/pull/9284.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9284/head:pull/9284

PR: https://git.openjdk.org/jdk/pull/9284