RFR: 8241710: NullPointerException while entering empty submenu with "arrow right"

[Ajit Ghaisas]

Bug : https://bugs.openjdk.java.net/browse/JDK-8241710

Root Cause : A menu can have empty submenu. This was not checked while processing RIGHT array key.

Fix : Added the null check for submenu. Added a unit test case which fails without fix and passes with it.

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Commit messages:
 - Fix NPE

Changes: https://git.openjdk.java.net/jfx/pull/161/files
 Webrev: https://webrevs.openjdk.java.net/jfx/161/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8241710
  Stats: 29 lines in 2 files changed: 20 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jfx/pull/161.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/161/head:pull/161

PR: https://git.openjdk.java.net/jfx/pull/161