RFR: JDK-8323543: NPE when table items are set to null [v2]

[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 one additional commit since the last revision:

  JDK-8323543: clean up code

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1329/files
  - new: https://git.openjdk.org/jfx/pull/1329/files/31b408c6..54e8668e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1329&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1329&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 4 del; 2 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