SET character_set_results=NULL is now working

sql/set_var.cc

@@ -1280,9 +1280,15 @@ bool sys_var_character_set::check(THD *thd, set_var *var)
   String str(buff,sizeof(buff), system_charset_info), *res;
 
   if (!(res=var->value->val_str(&str)))
-    res= &empty_string;
-
-  if (!(tmp=get_charset_by_csname(res->c_ptr(),MY_CS_PRIMARY,MYF(0))) &&
+  { 
+    if (!nullable)
+    {
+      my_error(ER_UNKNOWN_CHARACTER_SET, MYF(0), "NULL");
+      return 1;
+    }
+    tmp= NULL;
+  }
+  else if (!(tmp=get_charset_by_csname(res->c_ptr(),MY_CS_PRIMARY,MYF(0))) &&
       !(tmp=get_old_charset_by_name(res->c_ptr())))
   {
     my_error(ER_UNKNOWN_CHARACTER_SET, MYF(0), res->c_ptr());

sql/set_var.h

@@ -433,7 +433,9 @@ SHOW_TYPE type() { return SHOW_CHAR; }
 class sys_var_character_set :public sys_var_thd
 {
 public:
-  sys_var_character_set(const char *name_arg) :sys_var_thd(name_arg) {}
+  bool nullable;
+  sys_var_character_set(const char *name_arg) :sys_var_thd(name_arg) 
+  { nullable= 0; }
   bool check(THD *thd, set_var *var);
 SHOW_TYPE type() { return SHOW_CHAR; }
   bool check_update_type(Item_result type)
@@ -460,7 +462,8 @@ class sys_var_character_set_results :public sys_var_character_set
 {
 public:
   sys_var_character_set_results(const char *name_arg) :
-    sys_var_character_set(name_arg) {}
+    sys_var_character_set(name_arg) 
+    { nullable= 1; }
   void set_default(THD *thd, enum_var_type type);
   CHARSET_INFO **ci_ptr(THD *thd, enum_var_type type);
 };