RFR: 8279514: NPE on clearing value of IntegerSpinnerValueFactory

[Ajit Ghaisas]

A null check has been added to below SpinnerValueFactory classes in valueProperty() ChangeListeners-
- `IntegerSpinnerValueFactory`
- `LocalDateSpinnerValueFactory`
- `LocalTimeSpinnerValueFactory`
- `ListSpinnerValueFactory`

Added 5 tests that detect NPE for above 4 Spinner factories.
Only `DoubleSpinnerValueFactory` already had a valid null check - so the test for it passes before and after.

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Commit messages:
 - Add null checks to SpinnerValueFactories

Changes: https://git.openjdk.org/jfx/pull/865/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=865&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8279514
  Stats: 46 lines in 2 files changed: 46 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/865.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/865/head:pull/865

PR: https://git.openjdk.org/jfx/pull/865