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

[Carl Döbbelin]

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

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Commit messages:
 - JDK-8323543: adds similar safety test in TreeTableViewTest
 - JDK-8323543: Fixes Nullpointer in TableSkinUtils

Changes: https://git.openjdk.org/jfx/pull/1329/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1329&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323543
  Stats: 53 lines in 4 files changed: 41 ins; 11 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