RFR: 8287603: Avoid redundant HashMap.containsKey calls in NimbusDefaults.getDerivedColor

[Andrey Turbanov]

The method `javax.swing.plaf.nimbus.NimbusDefaults#getDerivedColor(String,String,float,float,float,int,boolean)` could be improved by usage of Map.putIfAbsent instead of separate `containsKey`/`get`/`put` calls. We known that HashMap `javax.swing.plaf.nimbus.NimbusDefaults#derivedColors` can contain only non-null values. And to check if `putIfAbsent` was successful or not, we can just compare result with `null`.
https://github.com/openjdk/jdk/blob/97bd4c255a319ce626a316ed211ef1fd7d0f1e14/src/java.desktop/share/classes/javax/swing/plaf/nimbus/Defaults.template#L713-L720
It makes code a bit cleaner and faster.

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Commit messages:
 - [PATCH] Avoid redundant HashMap.containsKey calls in NimbusDefaults

Changes: https://git.openjdk.java.net/jdk/pull/8482/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8482&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287603
  Stats: 8 lines in 1 file changed: 1 ins; 3 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8482.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8482/head:pull/8482

PR: https://git.openjdk.java.net/jdk/pull/8482