bean: fix the OfficeBean SDK example

Regression from 26b4f5be81 (java:use
System.arrayCopy to copy arrays, 2015-06-09), as the old code used to handle
the case when aArguments was null.

How to reproduce:

1) Source the SDK environment.
2) cd instdir/sdk/examples/DevelopersGuide/OfficeBean/
3) make OOoBeanViewer.run
4) Click new document... -> Text Document -> NullPointerException

Change-Id: I5521c7fd200f6c74c5f2665f2e4a99f0d6abfeeb
Reviewed-on: https://gerrit.libreoffice.org/24366
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>

bean/com/sun/star/comp/beans/OOoBean.java

@@ -1349,7 +1349,8 @@ xLayoutManager.showElement("private:resource/menubar/menubar");
             new com.sun.star.beans.PropertyValue[ nNumArgs + 1 ];
 
         // copy current arguments
-        System.arraycopy(aArguments, 0, aExtendedArguments, 0, nNumArgs);
+        if (aArguments != null)
+            System.arraycopy(aArguments, 0, aExtendedArguments, 0, nNumArgs);
 
         // add new argument
         aExtendedArguments[ nNumArgs ] = aArgument;