Fix: do use the empty string as the action name when no action is registred

lib/sasacore/process.ml

-(* Time-stamp: <modified the 13/03/2019 (at 10:01) by Erwan Jahier> *)
+(* Time-stamp: <modified the 20/03/2019 (at 13:07) by Erwan Jahier> *)
 
 type t = {
   pid : string;
@@ -44,7 +44,7 @@ let (make: bool -> Topology.node -> t) =
       with _ ->
         if custom_mode then
           failwith "Registering actions is mandatory in algorithms when using custom demon!"
-        else [""]
+        else ["a"]
     in
     let process = {
       pid = pid;