RFR: 8341281: Root TreeItem with null value breaks TreeTableView [v5]

[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 incrementally with two additional commits since the last revision:

 - Merge remote-tracking branch 'origin/8341281.RootTreeItem' into 8341281.RootTreeItem
 - Adding fix to root item listener

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1767/files
  - new: https://git.openjdk.org/jfx/pull/1767/files/dc0de460..70b5e34c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1767&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1767&range=03-04

  Stats: 10 lines in 1 file changed: 5 ins; 5 del; 0 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