core: use DUAL_TIMESTAMP_NULL where we can

src/core/main.c

@@ -1391,8 +1391,7 @@ int main(int argc, char *argv[]) {
 
                 /* clear the kernel timestamp,
                  * because we are not PID 1 */
-                kernel_timestamp.monotonic = 0ULL;
-                kernel_timestamp.realtime = 0ULL;
+                kernel_timestamp = DUAL_TIMESTAMP_NULL;
         }
 
         /* Initialize default unit */