RFR: 8341281: Root TreeItem with null value breaks TreeTableView

[Ziad El Midaoui]

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

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Commit messages:
 - Fixed issue with TreeTableView not showing the root's children when set to null

Changes: https://git.openjdk.org/jfx/pull/1767/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1767&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341281
  Stats: 5 lines in 1 file changed: 5 ins; 0 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