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

[Sergey Bylokhov]

On Sun, 27 Apr 2025 21:04:34 GMT, Andrey Turbanov <aturbanov at openjdk.org> wrote:

> `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`.

Marked as reviewed by serb (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/24907#pullrequestreview-2874193509