Missing values in conditions behave the same way as do normal values or answers.
If the condition includes 'categories - single answer' or 'categories - multiple answers' question types, then the option '(-1) Unanswered question' will appear between the responses, when setting a condition. Other missing values, such as 'Don't know', 'Invalid' behave in the same way. If you select them in the question's editor, they will display as an answer, which you can select when setting a condition.
However, if the condition includes text or numeric question type and you want to set a condition in case the respondent does not enter anything, then leave the field empty. Only two quotation marks will be shown in the condition.
The use of conditions is only available to users of the 2KA and 3KA private packages.