Add Fake Display when No Monitor is Plugged In

Basically install a dummy driver:

apt-get install xserver-xorg-video-dummy

Then write it in the /usr/share/X11/xorg.conf.d/xorg.conf (or possibly /etc/X11/xorg.conf) file (create one, if it does not exist):

* Before changing xorg.conf file, set up remote desktop server first, like “RustDesk” and create backup of original file.

Section "Device"
    Identifier  "Configured Video Device"
    Driver      "dummy"
    # Default is 4MiB, this sets it to 16MiB
    VideoRam    16384
EndSection

Section "Monitor"
    Identifier  "Configured Monitor"
    HorizSync 31.5-48.5
    VertRefresh 50-70
EndSection

Section "Screen"
    Identifier  "Default Screen"
    Monitor     "Configured Monitor"
    Device      "Configured Video Device"
    DefaultDepth 24
    SubSection "Display"
    Depth 24
    Modes "1024x800"
    EndSubSection
EndSection

Then restart your X session by logging out.

Source: External Link