RFR: 8341281: Root TreeItem with null value breaks TreeTableView [v2]

[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 two additional commits since the last revision:

 - Merge branch 'openjdk:master' into 8341281.RootTreeItem
 - Fixed issue with TreeTableView not showing the root's children when set to null

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1767/files
  - new: https://git.openjdk.org/jfx/pull/1767/files/c80550e4..a7747486

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1767&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1767&range=00-01

  Stats: 125100 lines in 321 files changed: 30056 ins; 52530 del; 42514 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