group_setupidlefiles: fall back to /dev/null if /dev/console is disabled

[yamt]

Summary

group_setupidlefiles: fall back to /dev/null if /dev/console is disabled

Impact

The warning may bother some users.

Testing

Tested with a modified nsh on esp32-devkitc.

 int main(int argc, FAR char *argv[])
  {
  +  int fd = open("/dev/null", O_RDWR);
  +  _info("hey _info %d\n", fd);
  +  write(1, "hey stdout\n", 11);
  +  write(2, "hey stderr\n", 11);