RFR: 8089398: [ChoiceBox, ComboBox] throws NPE on setting value on null selectionModel [v5]

[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 two additional commits since the last revision:

 - removed unneeded assert and added comment
 - removed unneeded asserts and added another NPE fix.

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/557/files
  - new: https://git.openjdk.java.net/jfx/pull/557/files/b064f61e..039b826b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=557&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=557&range=03-04

  Stats: 33 lines in 2 files changed: 20 ins; 11 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