RFR: 8089398: [ChoiceBox, ComboBox] throws NPE on setting value on null selectionModel [v2]

[Marius Hanl]

> This PR fixes 2 NPEs in Choice-and ComboBox, when the selection model is null.
> 
> ChoiceBox: 
> - Null check in **valueProperty()** listener
> 
> ComboBox:
> - Null check in **valueProperty()** listener
> - Null check in **ComboBoxListViewSkin#updateValue()**
> 
> The tests checks, that no NPE is printed to the console. They also checks, that the set value is still displayed (either in the ComboBox button cell or the ChoiceBox display label)

Marius Hanl has updated the pull request incrementally with two additional commits since the last revision:

 - removed now unused imports
 - Review adjustments

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/557/files
  - new: https://git.openjdk.java.net/jfx/pull/557/files/671cc70a..a47c0652

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=557&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=557&range=00-01

  Stats: 38 lines in 2 files changed: 17 ins; 19 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/557.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/557/head:pull/557

PR: https://git.openjdk.java.net/jfx/pull/557