RFR: 6603771: Nimbus L&F: Ctrl+F7 keybinding for Jinternal Frame throws a NPE.

[Prasanta Sadhukhan]

In Swingset2 JInternalFrame demo, focusing on any internalframes and pressing Ctrl+F7 and then any of up/down/left/arrow key to move the internal frames results in NPE in NimbusL&F.
It is because `JComponent.processKeyBinding() `calls `SwingUtilities.notifyAction` which calls `BasicDesktopPaneUI.actionPerformed` where it gets `Desktop.minOnScreenInsets` property to get the Insets and then uses insets.top/bottom/left/right value to move the internalframe but this property is not defined for Nimbus so deferencing Insets object results in NPE.

Fix is added to create a 0 insets incase some L&F do not define` Desktop.minOnScreenInsets` property.
No regression test is added as it can be easily checked by SwingSet2 JInternalFrame demo.

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Commit messages:
 - 6603771: Nimbus L&F: Ctrl+F7 keybinding for Jinternal Frame throws a NPE.

Changes: https://git.openjdk.org/jdk/pull/11605/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11605&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6603771
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11605.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11605/head:pull/11605

PR: https://git.openjdk.org/jdk/pull/11605