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

[Carl Döbbelin]

> 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

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1329/files
  - new: https://git.openjdk.org/jfx/pull/1329/files/f883a003..bc515d7d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1329&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1329&range=02-03

  Stats: 11 lines in 3 files changed: 7 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1329.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1329/head:pull/1329

PR: https://git.openjdk.org/jfx/pull/1329