RFR: JDK-8323543: NPE when table items are set to null [v6]

[Carl Döbbelin]

> This PR fixes a nullpointer in TableSkinUtils that occured when the Tables items were null.

Carl Döbbelin has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  JDK-8323543: change tests to JUnit4 for compatibility

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1329/files
  - new: https://git.openjdk.org/jfx/pull/1329/files/6bd6e18c..20cc6320

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1329&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1329&range=04-05

  Stats: 691 lines in 2 files changed: 12 ins; 290 del; 389 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