How to fool a laptop into thinking a monitor is connected?

Hello! I converted an old laptop with a broken screen into a home server, and it all works well except for one thing: when I reboot it (via ssh), if no screen is connected, it will get stuck and refuse to boot. as soon as I connect an HDMI monitor, the fans will start spinning and it will start booting as usual. Then I can remove the HDMI and it will work flawlessly. I don't know if this is a linux problem, a GRUB problem, or a firmware problem.
Any idea on how to solve this, or on how to fool it into thinking a screen is connected? The problem is not the lid switch as I removed the magnet from the screen, so it thinks the lid is always open

Thanks in advance!