Minor bug in Java interface

Dear bugpeople,

class Atom 

The constructor should not accept null as a valid input parameter. An Atom
with a null functor() causes a NullPointerException in
EXDROutputStream.write()


This may generalise to the point that no term should have a null functor.
Perhaps the fix should include a change in EXDROutputStream.write() to check
for null functors and return a nice error message.

Alternatively you could call it a programming error, but if so I think it's
worth highlighting it in the javadoc because it's not immediately obvious
when you get a NullPointerException in the middle of a write().

Hope this helps,

Nick