Use another field as a fallback when a merge field is blank

In some cases the first name field is a legal or proper name and the contact has provided us a preferred name. We would like to use the preferred name as the greeting for these contacts. However, since many contacts don't have both fields filled out we need the option for a fallback chain.

For example:

The greeting is set to: Hi {preferred_name}
If preferred_name is blank, then the greeting is: Hi {first_name}

and if first_name is also blank, the greeting becomes: Hi friend