examples/sched_ctx/two_cpu_contexts.c: using empty string instead of NULL pointer

examples/sched_ctx/two_cpu_contexts.c

@@ -72,8 +72,8 @@ int main(int argc, char **argv)
 		procs2[i] = procs[i+nprocs1];
 	}
 
-        /* create sched context 1 with default policy, by giving a NULL policy name */
-	unsigned sched_ctx1 = starpu_sched_ctx_create(procs1, nprocs1, "ctx1", STARPU_SCHED_CTX_POLICY_NAME, NULL, 0);
+        /* create sched context 1 with default policy, by giving a empty policy name */
+	unsigned sched_ctx1 = starpu_sched_ctx_create(procs1, nprocs1, "ctx1", STARPU_SCHED_CTX_POLICY_NAME, "", 0);
         /* create sched context 2 with a user selected policy name */
 	unsigned sched_ctx2 = starpu_sched_ctx_create(procs2, nprocs2, "ctx2", STARPU_SCHED_CTX_POLICY_NAME, "eager", 0);