RFR: 8089398: [ChoiceBox, ComboBox] throws NPE on setting value on null selectionModel [v6]

[Marius Hanl]

> This PR fixes multiple NPEs in Choice-and ComboBox, when the selection model is null.
> 
> ChoiceBox: 
> - Null check in **valueProperty()** listener
> 
> ComboBox:
> - Null check in **editableProperty()** listener
> - Null check in **valueProperty()** listener
> - Null check in **ComboBoxListViewSkin#updateValue()**
> - Null check in **ComboBoxListViewSkin#layoutChildren()**
> - Null check in **ComboBoxListViewSkin#createListView()**
> 
> ~~The tests checks, that no NPE is printed to the console.
> Some 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 one additional commit since the last revision:

  Fixed another NPE on skin creation when the cbx selection model is null

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/557/files
  - new: https://git.openjdk.java.net/jfx/pull/557/files/039b826b..40a30e08

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=557&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=557&range=04-05

  Stats: 14 lines in 2 files changed: 11 ins; 0 del; 3 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