Improve type stability within styled"" macro

This way, when interpolating, the type of the expression can be inferred
as an AnnotatedString, rather than a Union{String, AnnotatedString}.

Author: tecosaur

src/styledmarkup.jl

@@ -263,7 +263,7 @@ function addpart!(state::State, start::Int, expr, stop::Int)
                           AnnotatedString(String($str), vcat($annots, annotations($str)))
                       else
                           if isempty($str)
-                              ""
+                              AnnotatedString("")
                           else
                               AnnotatedString($str, $annots)
                           end