RFR: JDK-8320056 : [macos14] Add missing null checks to GetMousePositionWithPopup.java test

[Harshitha Onkar]

This test  passes on the latest version of macOS (14.2). The reason as to why this passes on the latest version of macOS is the order in which native Mouse Moved event is received. Details and native event logs added to the JBS issue.

While looking into it noticed missing null checks in test hence adding them as part of this PR.

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Commit messages:
 - null checks added

Changes: https://git.openjdk.org/jdk/pull/17380/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17380&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320056
  Stats: 8 lines in 1 file changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/17380.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17380/head:pull/17380

PR: https://git.openjdk.org/jdk/pull/17380