RFR: 8341286: TreeView: visual artifacts when setting new root with null value

[Ziad El Midaoui]

Fix for phantom disclosure nodes appearing when root is null

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Commit messages:
 - Fix for issue with null root in TreeView
 - Merge branch 'openjdk:master' into 8341286.TreeViewArtifacts
 - Partial fix

Changes: https://git.openjdk.org/jfx/pull/1838/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1838&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341286
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1838.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1838/head:pull/1838

PR: https://git.openjdk.org/jfx/pull/1838