RFR: 8341281: Root TreeItem with null value breaks TreeTableView [v4]

[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 one additional commit since the last revision:

  Minor change : new line added

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1767/files
  - new: https://git.openjdk.org/jfx/pull/1767/files/bf0f0a13..dc0de460

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1767&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1767&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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