Empty body should be nil irep

and not empty code block. Otherwise it over-shadows the actual definition.

Author: petr-bauch

gnat2goto/driver/goto_utils.ads

@@ -64,7 +64,7 @@ package GOTO_Utils is
                                           Subprog_Body : Irep;
                                           A_Symbol_Table : in out Symbol_Table)
      with Pre => Kind (Subprog_Type) = I_Code_Type
-     and Kind (Subprog_Body) = I_Code_Block;
+     and Kind (Subprog_Body) in I_Nil | I_Code_Block;
    --  Insert the subprogram specification into the symbol table
 
    procedure New_Type_Symbol_Entry (Type_Name : Symbol_Id; Type_Of_Type : Irep;

gnat2goto/driver/tree_walk.adb

@@ -5457,7 +5457,7 @@ package body Tree_Walk is
         Do_Subprogram_Specification (N);
       Subprog_Name : constant Symbol_Id :=
         Intern (Unique_Name (Defining_Unit_Name (N)));
-      Default_Body : Irep := Make_Code_Block (Sloc (N));
+      Default_Body : Irep := Make_Nil (Sloc (N));
    begin
       if List_Length (Parameter_Specifications (N)) = 1 and
       not (Kind (Get_Return_Type (Subprog_Type)) = I_Void_Type)