RFR: 8341281: Root TreeItem with null value breaks TreeTableView [v6]

[Ziad El Midaoui]

> When the Root TreeItem is set to null, need to relayout to show the children items

Ziad El Midaoui has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Merge branch 'openjdk:master' into 8341281.RootTreeItem
 - Merge remote-tracking branch 'origin/8341281.RootTreeItem' into 8341281.RootTreeItem
 - Minor change : new line added
 - Adding fix to root item listener
 - Merge remote-tracking branch 'origin/8341281.RootTreeItem' into 8341281.RootTreeItem
 - Merge branch 'openjdk:master' into 8341281.RootTreeItem
 - Headfull Test added
 - Fixed issue with TreeTableView not showing the root's children when set to null

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1767/files
  - new: https://git.openjdk.org/jfx/pull/1767/files/70b5e34c..5c55cc0d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1767&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1767&range=04-05

  Stats: 313 lines in 20 files changed: 303 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jfx/pull/1767.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1767/head:pull/1767

PR: https://git.openjdk.org/jfx/pull/1767