Missing values behave similarly to normal values or answers in the conditions.
If a 'Categories - Single answer' or 'Categories - Multiple answers' question type is included in a condition, the option (-1) Not answered will always appear among the answers when setting the condition. Other missing values such as Don't know, Not applicable ... behave in the same way. If you set missing values in a question, they will be shown as an answer that you can select when setting the condition.
However, if you include a Text or Number question type in the condition and you want to set the condition in case the respondent does not enter anything, then leave the field blank. Only the quotation mark will appear in the condition display.