RFR: 8251941: ListCell: visual artifact when items contain null values

[Jeanette Winzenburg]

The issue describes the makroscopic effect/s, namely content showing in cells that are off range.

The base reason is missing cleanup of the cell on transition from not-empty to empty when the old item is a null
contained in the items. Fixed by changing the logic to cope with that special case. Added tests that fail before and
pass after the fix.

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Commit messages:
 - 8251941: ListCell: visual artifact when items contain null values

Changes: https://git.openjdk.java.net/jfx/pull/288/files
 Webrev: https://webrevs.openjdk.java.net/jfx/288/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251941
  Stats: 90 lines in 2 files changed: 87 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/288.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/288/head:pull/288

PR: https://git.openjdk.java.net/jfx/pull/288