Don't return null when being unable to deserialize a DropMessage's data field

Hopefully it won't break stuff and/or destroys the world 
- one less possibility of having NREs though!
> Just curious - why did you change these lines of code?
Less code is better code, or less bug-attrackting, imho.

> `DropMessage<>` instead of `DropMessage<ModelObject>`
NetBeans suggested me that this was possible - and well, less code as well :)