Define null_id_vec for readability. Fixes #993.

vunit/vhdl/data_types/src/id_pkg.vhd

@@ -22,6 +22,7 @@ package id_pkg is
   end record;
   constant null_id : id_t := (p_data => null_ptr);
   type id_vec_t is array (integer range <>) of id_t;
+  constant null_id_vec : id_vec_t := (1 to 0 => null_id);
 
   -- root_id is a nameless and predefined ID that is the parent to
   -- all user created top-level IDs (no parent was specified at creation)
@@ -348,7 +349,7 @@ package body id_pkg is
   begin
     if id = null_id then
       null_id_failure("get_lineage");
-      return (1 to 0 => null_id);
+      return null_id_vec;
     elsif id = root_id then
       return (1 => root_id);
     end if;