combining two strings

// what's a better way?

let fullName (first,last) =
    match (first, last) with
        "",_ | null,_ -> first
        | _,null | _,"" -> last
        | _ -> sprintf "%s.%s" first last

           let isnullorempty = function
             | null -> true
             | "" -> true
             | _ -> false

           let fullname(first,last) =
             match isnullorempty first with
             | true -> last
             | false -> match isnullorempty last with
                           | true -> first
                           | false -> sprintf "%s.%s" first last

Matt offered this solution too:
http://gist.github.com/474421