RFR: 8187145: (Tree)TableView with null selectionModel: throws NPE on sorting [v13]

[Andy Goryachev]

> Setting a null selection model in TableView and TreeTableView produce NPE on sorting (and probably in some other situations) because the check for null is missing in several places. 
> 
> Setting a null selection model is a valid way to disable selection in a (tree)table.
> 
> There is also a similar issue with ListView [JDK-8279640](https://bugs.openjdk.org/browse/JDK-8279640).
> 
> changes:
> - added check for null selection model where it was missing
> - clear focused row index on setting null selection model in TreeTableView
> - clear selected cells on setting a new selection model

Andy Goryachev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 20 commits:

 - Merge remote-tracking branch 'origin/master' into 8187145.null.selection.model
 - Merge remote-tracking branch 'origin/master' into 8187145.null.selection.model
 - 8187145: cleanup
 - 8187145: also tree table view
 - 8187145: whitespace
 - Merge remote-tracking branch 'origin/master' into 8187145.null.selection.model
 - 8187145: review comments
 - 8187145: github
 - Merge remote-tracking branch 'origin/master' into 8187145.null.selection.model
 - Merge remote-tracking branch 'origin/master' into 8187145.null.selection.model
 - ... and 10 more: https://git.openjdk.org/jfx/compare/bb98d886...2e19c490

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Changes: https://git.openjdk.org/jfx/pull/876/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=876&range=12
  Stats: 471 lines in 15 files changed: 378 ins; 12 del; 81 mod
  Patch: https://git.openjdk.org/jfx/pull/876.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/876/head:pull/876

PR: https://git.openjdk.org/jfx/pull/876