I think a better idea is to create a formula field in the admin section of Marketo. Call it Letter Salutation or something along those lines. And start off with IF First Name is not empty, then Hello {{lead.First Name}} ... If Last Name is not empty, then Hello {{lead.Last Name}} ... If Full name is empty, then Greetings.
That way you go ahead and put the comma directly into the field and prevent any confusion with how that will work. Then all you need to do is add
{{lead.Letter Saluation}} to the top of your emails.