RFR: 8274640: Cleanup unnecessary null comparison before instanceof check in java.desktop

[Andrey Turbanov]

Updated code checks both non-null and instance of a class in java.desktop module classes.
The checks and explicit casts could also be replaced with pattern matching for the instanceof operator. 
Similar cleanups
1. [JDK-8273484](https://bugs.openjdk.java.net/browse/JDK-8273484) java.naming
2. [JDK-8258422](https://bugs.openjdk.java.net/browse/JDK-8258422) java.base

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Commit messages:
 - [PATCH] Cleanup unnecessary null comparison before instanceof check in java.desktop
 - [PATCH] Cleanup unnecessary null comparison before instanceof check in java.desktop
 - [PATCH] Cleanup unnecessary null comparison before instanceof check in java.desktop
 - [PATCH] Cleanup unnecessary null comparison before instanceof check in java.desktop
 - [PATCH] Cleanup unnecessary null comparison before instanceof check in java.desktop
 - [PATCH] Cleanup unnecessary null comparison before instanceof check in java.desktop
 - [PATCH] Cleanup unnecessary null comparison before instanceof check in java.desktop

Changes: https://git.openjdk.java.net/jdk/pull/5482/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5482&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274640
  Stats: 543 lines in 122 files changed: 0 ins; 114 del; 429 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5482.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5482/head:pull/5482

PR: https://git.openjdk.java.net/jdk/pull/5482