Fix to NullDebugCounter makes unit tests pass

...main/java/net/floodlightcontroller/core/OFSwitchBase.java

...a/net/floodlightcontroller/debugcounter/DebugCounter.java

...t/floodlightcontroller/debugcounter/NullDebugCounter.java

...tcontroller/devicemanager/internal/DeviceManagerImpl.java

...ntroller/linkdiscovery/internal/LinkDiscoveryManager.java

...va/net/floodlightcontroller/topology/TopologyManager.java