Better error messages for not nullable

SET character_set_xxx=NULL
and
SET collation_xxx=NULL

mysql-test/r/variables.result

@@ -233,6 +233,10 @@ set character_set_client=UNKNOWN_CHARACTER_SET;
 ERROR 42000: Unknown character set: 'UNKNOWN_CHARACTER_SET'
 set collation_connection=UNKNOWN_COLLATION;
 ERROR HY000: Unknown collation: 'UNKNOWN_COLLATION'
+set character_set_client=NULL;
+ERROR 42000: Variable 'character_set_client' can't be set to the value of 'NULL'
+set collation_connection=NULL;
+ERROR 42000: Variable 'collation_connection' can't be set to the value of 'NULL'
 set global autocommit=1;
 ERROR HY000: Variable 'autocommit' is a SESSION variable and can't be used with SET GLOBAL
 select @@global.timestamp;

mysql-test/t/variables.test

@@ -137,6 +137,10 @@ set GLOBAL storage_engine=DEFAULT;
 set character_set_client=UNKNOWN_CHARACTER_SET;
 --error 1273
 set collation_connection=UNKNOWN_COLLATION;
+--error 1231
+set character_set_client=NULL;
+--error 1231
+set collation_connection=NULL;
 --error 1228
 set global autocommit=1;
 --error 1238

sql/set_var.cc

@@ -1678,7 +1678,10 @@ bool sys_var_collation::check(THD *thd, set_var *var)
   String str(buff,sizeof(buff), system_charset_info), *res;
 
   if (!(res=var->value->val_str(&str)))
-    res= &my_empty_string;
+  {
+    my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), name, "NULL");
+    return 1;
+  }
 
   if (!(tmp=get_charset_by_name(res->c_ptr(),MYF(0))))
   {
@@ -1700,7 +1703,7 @@ bool sys_var_character_set::check(THD *thd, set_var *var)
   { 
     if (!nullable)
     {
-      my_error(ER_UNKNOWN_CHARACTER_SET, MYF(0), "NULL");
+      my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), name, "NULL");
       return 1;
     }
     tmp= NULL;