It is not possible to set conditional branching logic for merge fields, such as "If field A is blank, output field B or a fixed value in the merge field". Create a formula field in Salesforce and set the formula field as a merge field.
As an exception, the True/False values of checkbox fields can be replaced with other values.
e.g. {!Opportunity.IsDelivery__c, Yes;No}
Instruction manual
Replace True/False in Checkbox Fields with Other Values