KBF-37 set default arg = nullptr to task::start<>()

include/kbf/task.h

@@ -20,7 +20,7 @@ namespace kbf {
     namespace task {
         template<class T>
         std::shared_ptr<Task>
-        start(const std::string &name, void *arg, uint32_t stackSize = 2048, uint32_t priority = 5);
+        start(const std::string &name, void *arg = nullptr, uint32_t stackSize = 2048, uint32_t priority = 5);
     }
 
     /**