Null values in Salesforce CPQ Price Rules and Product Rules

To indicate you want to test whether or not a field value is null for a given price condition or error condition, the corresponding Filter Value field should be left blank. You can set a null value to fields by keeping the Value field on price actions or the Filter Value field on product actions blank.

In Salesforce CPQ, only a blank field is treated as null; including any text (including 0, "", null, etc) in the field value results in using that text as a string and/or number to evaluate against.