RFR: 8341286: TreeView: visual artifacts when setting new root with null value [v3]

[duke]

On Tue, 29 Jul 2025 15:55:02 GMT, Ziad El Midaoui <zelmidaoui at openjdk.org> wrote:

>> The issue is that when we use null value for root item in a TreeView and then expand the root or change it dynamically there are phantom disclosure nodes that appear incorrectly. 
>> The issue was caused by this line `if (getSkinnable().isEmpty()) return;` in `TreeCellSkin#updateDisclosureNode()` as it prevents the method from running unless the cell's value is not empty and it caused the creation of disclosure nodes visually without any logic behind.
>
> Ziad El Midaoui has updated the pull request incrementally with one additional commit since the last revision:
> 
>   minor change

@Ziad-Mid 
Your change (at version 95997e00bf8d8ac966ca79826df0e8dbc3712a8c) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/jfx/pull/1838#issuecomment-3141927290