Set scheduler_coro_add() arg value to NULL by default

wrapper.pyx

@@ -109,7 +109,7 @@ cdef void scheduler_init(scheduler_t *scheduler, unsigned short num_workers=sysc
   for i in range(scheduler.num_workers):
     worker_init(&scheduler.workers[i], scheduler)
 
-cdef void scheduler_coro_add(scheduler_t *scheduler, coro_function_t task, void *arg) nogil:
+cdef void scheduler_coro_add(scheduler_t *scheduler, coro_function_t task, void *arg=NULL) nogil:
   cdef worker_t *main_worker = scheduler.workers
   main_worker.deque.push_back(coro_new(&main_worker.switcher, task, arg))
 
@@ -241,7 +241,7 @@ def main():
     scheduler_init(&s)
 
     for i in range(500):
-      scheduler_coro_add(&s, task, NULL)
+      scheduler_coro_add(&s, task)
 
     scheduler_run(&s)
     scheduler_destroy(&s)