RFR: 8356175: Remove unnecessary Map.get from XWM.getInsets

[Andrey Turbanov]

Cleanup code to use Map.putIfAbsent. It's clearer and faster.

-------------

Commit messages:
 - [PATCH] Remove unnecessary Map.get from XWM.getInsets

Changes: https://git.openjdk.org/jdk/pull/24906/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24906&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356175
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24906.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24906/head:pull/24906

PR: https://git.openjdk.org/jdk/pull/24906