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

[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
- clearing focused row index on setting null selection model in TreeTableView

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Commit messages:
 - 8187145: whitespace
 - 8187145: clear focus
 - 8187145: tree table view
 - 8187145: fall through
 - 8187145: check for null selection model

Changes: https://git.openjdk.org/jfx/pull/876/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=876&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8187145
  Stats: 182 lines in 11 files changed: 89 ins; 10 del; 83 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