fix _subtype_info

Author: Maximilian-Stefan-Ernst

src/frontend/specification/Sem.jl

@@ -608,16 +608,17 @@ end
 # pretty printing
 ##############################################################
 
-_subtype_info(::Sem) = ""
-_subtype_info(::SemFiniteDiff) = " : Finite Difference Approximation"
+_subtype_info(::Sem) = nothing
+_subtype_info(::SemFiniteDiff) = "Finite Difference Approximation"
 
 function Base.show(io::IO, sem::AbstractSem)
     # if not specified, use compact printing for larger models
     if !haskey(io, :compact) && length(loss_terms(sem)) >= 5
         io = IOContext(io, :compact => true)
     end
     print(io, "Structural Equation Model")
-    print(io, _subtype_info(sem))
+    si = _subtype_info(sem)
+    isnothing(si) || print(io, " : "*si)
     print("\n")
     print(io, "- Loss Functions \n")
     for term in loss_terms(sem)