RFR: 8357687: Remove unnecessary Map.containsKey from GraphicsEnvironment implementations

[Andrey Turbanov]

`devices` contains only non-null values. It means instead of separate `containsKey` call we can just call `HashMap.get` and then compare result with `null`.

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Commit messages:
 - [PATCH] Remove unnecessary Map.containsKey from CGraphicsEnvironment.initDevices
 - [PATCH] Remove unnecessary Map.containsKey from X11GraphicsEnvironment.initDevices

Changes: https://git.openjdk.org/jdk/pull/24907/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24907&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357687
  Stats: 6 lines in 2 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/24907.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24907/head:pull/24907

PR: https://git.openjdk.org/jdk/pull/24907