RFR: JDK-8323543: NPE when table items are set to null [v4]

[Andy Goryachev]

On Thu, 11 Jan 2024 18:53:16 GMT, Carl Döbbelin <duke at openjdk.org> wrote:

>> This PR fixes a nullpointer in TableSkinUtils that occured when the Tables items were null.
>
> Carl Döbbelin has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - JDK-8323543: refactors tests and adds accessible attribute test for treetable
>  - JDK-8323543: remove unneeded comment

FYI, pushing another commit requires re-review (easy in this particular case).

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Marked as reviewed by angorya (Reviewer).

PR Review: https://git.openjdk.org/jfx/pull/1329#pullrequestreview-1816309708