RFR: 8267392: ENTER key press on editable TableView throws NPE

[Marius Hanl]

**Note: I reported the bug already, waiting for approval. Internal tracking id: 9070318. I will update the title as soon as the ticket is created.**

This PR is fixing a NP, which is thrown when you press ENTER on an editbale table, after it is initially shown.

When pressing ENTER, **TableViewBehaviorBase#activate** is retrieving the current focused row (**getFocusedCell()**) and from there the corresponding table column.
This is null, when a table is initially shown. It can also be null, when the items from the underlying table are changed (e.g. **setItems()**) or when **getFocusModel().focus(row)** is used. 
Therefore, null is a valid value and we should guard against it.

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Commit messages:
 - Fixed NP when ENTER is pressed on an editable table

Changes: https://git.openjdk.java.net/jfx/pull/505/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=505&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267392
  Stats: 39 lines in 3 files changed: 33 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/505.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/505/head:pull/505

PR: https://git.openjdk.java.net/jfx/pull/505