For example, let's say you want to start your email with "dear first name", if one of your constituents doesn't have a first name, you could enter your fallback as "friend" so that those with blank fields would have their email say "dear friend" instead of "dear ,"
We have built this into our database using Name Formats. Example: all constituents have an "Individual Informal Salutation" name format, which is built as preferred first name but if none then use first name. I imagine you could do something similar regarding "friend"?