RFR: 8279640: ListView with null SelectionModel/FocusModel throws NPE [v5]

[Marius Hanl]

> This PR fixes a bunch of NPEs when a null `SelectionModel` or `FocusModel` is set on a `ListView`.
> 
> The following NPEs are fixed (all are also covered by exactly one test case):
> NPEs with null selection model:
> - Mouse click on a `ListCell`
> - SPACE key press
> - KP_UP (arrow up) key press
> - HOME key press
> - END key press
> - BACK_SLASH + CTRL key press
> 
> NPEs with null focus model:
> - SPACE key press
> - Select an items: getSelectionModel().select(1)
> - Clear-Select an item and add one after: `listView.getSelectionModel().clearAndSelect(1); listView.getItems().add("3");`

Marius Hanl has updated the pull request incrementally with one additional commit since the last revision:

  Do selection/focus logic after the null check
  
  No need to prepare selection/focus stuff just to return later as one of them is null

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/711/files
  - new: https://git.openjdk.org/jfx/pull/711/files/5f61fe0a..2cc2bd16

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=711&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=711&range=03-04

  Stats: 29 lines in 2 files changed: 16 ins; 12 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/711.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/711/head:pull/711

PR: https://git.openjdk.org/jfx/pull/711