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