A source of zeros used mainly for testing. If you really want a constant source of zeros, try using the Constant Source block.
When a flowgraph has no hardware devices, a Throttle is needed to limit the CPU utilization. Since the "main" data flow is via messages, the Throttle is fed with a Null Source. This file can be found at 
- C++ files
- Header files
- Public header files
- Block definition