RFR: 8345398: Avoid redundant Properties.containsKey call in Cursor.getSystemCustomCursor

[Andrey Turbanov]

`Properties` doesn't allow `null` values.
We can replace containsKey+getProperty with getProperty+null check.
It's clearer and a bit faster.

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Commit messages:
 - [PATCH] Avoid redundant Properties.containsKey calls in Cursor.getSystemCustomCursor

Changes: https://git.openjdk.org/jdk/pull/21824/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21824&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345398
  Stats: 7 lines in 1 file changed: 2 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21824.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21824/head:pull/21824

PR: https://git.openjdk.org/jdk/pull/21824