RFR: 8283383: [macos] a11y : Screen magnifier shows extra characters (0) at the end JButton accessibility name

[Alexander Zuev]

Add the accessibilityValue implementation to the affected classes to return NULL value
Reverse the default initialization of MenuItemAccessibility if no parent is specified

-------------

Commit messages:
 - 8283383: [macos] a11y : Screen magnifier shows extra characters (0) at the end JButton accessibility name

Changes: https://git.openjdk.java.net/jdk/pull/9044/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9044&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8283383
  Stats: 29 lines in 5 files changed: 22 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9044.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9044/head:pull/9044

PR: https://git.openjdk.java.net/jdk/pull/9044