RFR: 8244418: MenuBar: IOOB exception on requestFocus on empty bar

[Ajit Ghaisas]

Issue :
https://bugs.openjdk.java.net/browse/JDK-8244418

Root Cause :
Incorrect assumption about menu list size.

Fix :
Added check for empty menu list before trying to access it.

Test :
Added a unit test that fails before fix and passes after it.

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Commit messages:
 - 8244418_fix

Changes: https://git.openjdk.java.net/jfx/pull/216/files
 Webrev: https://webrevs.openjdk.java.net/jfx/216/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244418
  Stats: 28 lines in 2 files changed: 22 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/216.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/216/head:pull/216

PR: https://git.openjdk.java.net/jfx/pull/216