It might be due to the version of NocoBase you’re using. This option is only available in more recent versions. I have this option in my environment. You could try deploying a more recent version, but make sure to do this in a sandbox environment to avoid any issues. If updating isn’t an option, another way to achieve this is by using a variable (if you have the workflow variable plugin installed).
By using a variable node, you can declare a node variable and set its value to null. You can then use this variable to assign its value to a field in an update node.
There is one more way I can think of to solve this problem, but it requires two additional plugins (one of which has a cost). However, if I were in your position, I would try to find a way to correct the update record to properly assign the null value.