The Linux kernel itself will use the serial console by default if there's no VGA device:

    If no console device is specified, the first device found capable of
    acting as a system console will be used. At this time, the system
    first looks for a VGA card and then for a serial port. So if you don't
    have a VGA card in your system the first serial port will automatically
    become the console.

https://www.kernel.org/doc/Documentation/admin-guide/serial-...