Activate root account without password in preseed file

[Kyras]

I need to create minimal preseed file that will activate root account without password.
Preseed file page does not specify all configuration values, but only mentions that, it is possible to activate root account but requires password.

Disclaimer:
I understand risks of root account without password, but this preseed file is required for VM contextualisation which requires active root without password from fresh installation
and will automatically set the password at the first boot.

[Capitain_Jack]

I guess you should not do it, a less unsecured method would be instead of no root password you can assign a user at sudoers file to not be asked for a password, than issue the sudo su command before what you want executed as root, of course you already said you understand the risks.