RFR: 8284676: TreeTableView loses sort ordering when applied on empty table [v2]

[Ambarish Rapte]

> Issue: In a TreeTableView if root item has no children then TreeTableView does not retain if a sort order is set.
> 
> Cause: This is a regression of fix for [JDK-8256283](https://bugs.openjdk.org/browse/JDK-8256283), which returned false from `SortPolicy.call()` function if root item has no children. This function is called from `TreeTableView.sort()` line #1863. When the returned value is false then line #1885 gets executed, which removes the sort order that was added.
> 
> Fix: The fix for [JDK-8256283](https://bugs.openjdk.org/browse/JDK-8256283) is modified to return true instead. Added a test that fails without and passes with fix.

Ambarish Rapte has updated the pull request incrementally with one additional commit since the last revision:

  review comment: correct test name

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/825/files
  - new: https://git.openjdk.org/jfx/pull/825/files/5768d03c..39353ce3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=825&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=825&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/825.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/825/head:pull/825

PR: https://git.openjdk.org/jfx/pull/825