RFR: 8332431: NullPointerException in JTable of SwingSet2

[Prasanta Sadhukhan]

Issue is observed in JTable demo in SwingSet2 whereby if we set the focus on a table cell (or click on a table cell) and Press Ctrl+F1 (show/hide tooltip) on a cell of JTable then NullpointerException is seen

> Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "java.awt.event.MouseEvent.getLocationOnScreen()" because "this.mouseEvent" is null 

This is because ToolTip associated with JTable demo button at top of SwingSet2 has MouseMotion Listener which causes [mouseEvent to be null](https://github.com/openjdk/jdk/blame/da3001daf79bf943d6194d9fd60250d519b9680d/src/java.desktop/share/classes/javax/swing/ToolTipManager.java#L573) when mouse exits the demo button area and enter the table cells area where show/hide tooltip causes showTipWindow to be called trying to access `mouseEvent` which is null.
Fix is made to check for null in this kind of cases..No regression test is added as it can be checked easily with SwingSet2..

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Commit messages:
 - 8332431: NullPointerException in JTable of SwingSet2

Changes: https://git.openjdk.org/jdk/pull/19379/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19379&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332431
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19379.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19379/head:pull/19379

PR: https://git.openjdk.org/jdk/pull/19379