Map null callback objects to null pointers

[pepijnve]

When passing null as value for a callback parameter to a native function a native closure gets created which wraps a null callable. A non-null pointer then gets passed to the native code. When the java closure gets invoke this triggers a null pointer exception at https://github.com/jnr/jnr-ffi/blob/master/src/main/java/jnr/ffi/provider/jffi/NativeClosureProxy.java#L57
Additionally this can end up changing the semantics of the function call since a non-null parameter is passed where null was intended. This is what's happening in #61.
This PR adds a null check in the 'to native' conversion of delegate types that returns a null Pointer instance.

[headius]

Nice find. You're right, I can name several common C functions that change their behavior if passed a non-null callback.