Did you try passwd -d root? Most likely, this will do what you want.
You can also manually edit /etc/shadow: (Create a backup copy. Be sure that you can log even if you mess up, for example from a rescue system.) Search for "root". Typically, the root entry looks similar to
root:$X$SK5xfLB1ZW:0:0...
There, delete the second field (everything between the first and second colon):
root::0:0...
Some systems will make you put an asterisk (*) in the password field instead of blank, where a blank field would allow no password (CentOS 8 for example)
root:*:0:0...
Save the file, and try logging in as root. It should skip the password prompt. (Like passwd -d, this is a "no password" solution. If you are really looking for a "blank password", that is "ask for a password, but accept if the user just presses Enter", look at the manpage of mkpasswd, and use mkpasswd to create the second field for the /etc/shadow.)