fixed NullPointerException for arraycolls with null elements

[dnet]

If an arraycoll had a null element, the software crashed like below

Exception in thread "main" java.lang.NullPointerException
        at org.unsynchronized.arraycoll.toString(arraycoll.java:45)
        at org.unsynchronized.arrayobj.toString(arrayobj.java:31)
        at org.unsynchronized.jdeserialize.dump_Instance(jdeserialize.java:329)
        at org.unsynchronized.jdeserialize.dump(jdeserialize.java:961)
        at org.unsynchronized.jdeserialize.main(jdeserialize.java:1187)

This pull request fixes this, the string null is printed instead of calling .toString() on (and thus dereferencing) null object references.

[dnet]

Is there an issue with this PR? What's the reason behind not merging it?

[dnet]

Ping?