If you would like to replace the variable tag (in email template ) with a default value if there is “NULL” value in that field.
For Example : First name variable tag – If there is a value in first name field of prospect than it should display “Hello John”. If there is null value than it should display “Hello Sir/ Ma’am”.
You can check the checkbox “Set a default mail merge value to be used when this field doesn’t have a value” and can set the default value in the first name field.
Below are the steps :
1. Navigate to Administration | Configure Fields | Prospect Fields.
2. Find the field you want to work with and click Edit.
3. Mark Set a default mail merge value to be used when this field doesn’t have a value.
4. Enter Default Mail Merge Value, then save the field.
Article for reference : https://help.salesforce.com/articleView?id=pardot_add_default_mail_merge_values.htm&type=0