We have some background processing on form and in the result, some of the form fields can become null. So how should information message about this action look like? The best option we have "The value of X was set to blank".
If I am understanding your question, there are some fields on your form that when information is entered, override some other fields that are no longer needed (for example, use saved phone number vs. enter a new one)
Second if the user has already entered input, you probably do not want to clear the value from the disabled field and instead just ignore that field when submitting. That way if the user were to change their mind, they could go back and continue editing the field without having to type everything all over again.
If I understand your question correctly, you have a form with a set of fields and on saving the form, you are processing it in a way that one or more fields are set to a NULL value.
It would be interesting to know why you want to set the values to NULL in the first place. But I can see a couple of possibilities here.
If you are setting blank fields or the fields which the users haven't filled to NULL - you can have a field placeholder text that can say something like for e.g. - Enter a value (Default NULL)
If you are overriding the user input to NULL - I don't believe it's a good practice. If the entered value is invalid, you can always validate and highlight the field. But there's no reason to override a valid user input, you can just avoid the field instead.