Fix NullPointerException in command handlers

Fix NullPointerException in command handlers

The active part can be null if all perspectives are closed. The handlers
isEnabled() method can then be invoked when using the workbench main
menu (e.g. File > Exit).

Change-Id: I76df9320238ef5e02aa0ee56a4b12dbb494cfe79
Reviewed-on: https://git.eclipse.org/r/9570
Tested-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>