RFR: 8279514: NPE on clearing value of IntegerSpinnerValueFactory [v2]

[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.

Ajit Ghaisas has updated the pull request incrementally with one additional commit since the last revision:

  Simplify null asserts

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/865/files
  - new: https://git.openjdk.org/jfx/pull/865/files/6d27071d..b04a79a2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=865&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=865&range=00-01

  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 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