terminal-util: handle the case where no system console is active (#40630)

/dev/console might have no backing driver, in which case
/sys/class/tty/console/active is empty. Unlike get_kernel_consoles()
resolve_dev_console() currently proceeds with empty devnode, resulting
in setup_input() -> acquire_terminal() emitting -EISDIR as we're trying
to open /dev/. Let's catch this and report -ENXIO.