Fix Bug #21328 mysqld issues warnings on ALTER CSV table to MyISAM

mysql-test/r/csv.result:
  update result file
mysql-test/r/log_tables.result:
  log_tables test contains alter of a CSV table with
  NULLs which results in warnings. In fact this is how
  the bug noticed. So, now when it is fixed we should
  update result file.
mysql-test/t/csv.test:
  add a test for the bug
storage/csv/ha_tina.cc:
  We should write 0 to the data file in the
  case we meet NULL. CSV does not support
  NULL values internally and we shouldn't
  distinguish them from 0 when writing a row
  (the alternative is to implement full NULL
  support). Otherwise other routines (such as
  Field::check_int() become confused).
  In 5.0 NULLs are stored as zeroes. In 5.1 it
  somehow turned into empty string. Which is wrong.

mysql-test/r/csv.result

@@ -5223,3 +5223,11 @@ check table bug22080_3;
 Table	Op	Msg_type	Msg_text
 test.bug22080_3	check	error	Corrupt
 drop tables bug22080_1,bug22080_2,bug22080_3;
+CREATE TABLE `bug21328` (
+`col1` int(11) DEFAULT NULL,
+`col2` int(11) DEFAULT NULL,
+`col3` int(11) DEFAULT NULL
+) ENGINE=CSV;
+insert into bug21328 values (1,NULL,NULL);
+alter table bug21328 engine=myisam;
+drop table bug21328;

mysql-test/r/log_tables.result

@@ -111,9 +111,6 @@ slow_log	CREATE TABLE `slow_log` (
 ) ENGINE=CSV DEFAULT CHARSET=utf8 COMMENT='Slow log'
 alter table mysql.general_log engine=myisam;
 alter table mysql.slow_log engine=myisam;
-Warnings:
-Warning	1366	Incorrect integer value: '' for column 'last_insert_id' at row 0
-Warning	1366	Incorrect integer value: '' for column 'insert_id' at row 0
 show create table mysql.general_log;
 Table	Create Table
 general_log	CREATE TABLE `general_log` (

mysql-test/t/csv.test

@@ -1605,3 +1605,17 @@ check table bug22080_2;
 check table bug22080_3;
 
 drop tables bug22080_1,bug22080_2,bug22080_3;
+
+#
+# Bug #21328 mysqld issues warnings on ALTER CSV table to MyISAM
+#
+
+CREATE TABLE `bug21328` (
+  `col1` int(11) DEFAULT NULL,
+  `col2` int(11) DEFAULT NULL,
+  `col3` int(11) DEFAULT NULL
+) ENGINE=CSV;
+
+insert into bug21328 values (1,NULL,NULL);
+alter table bug21328 engine=myisam;
+drop table bug21328;

storage/csv/ha_tina.cc

@@ -542,14 +542,16 @@ int ha_tina::encode_quote(byte *buf)
     const char *end_ptr;
 
     /*
-      Write an empty string to the buffer in case of a NULL value.
+      CSV does not support nulls. Write quoted 0 to the buffer. In fact,
+      (*field)->val_str(&attribute,&attribute) would usually return 0
+      in this case but we write it explicitly here.
       Basically this is a safety check, as no one ensures that the
       field content is cleaned up every time we use Field::set_null()
       in the code.
     */
     if ((*field)->is_null())
     {
-      buffer.append(STRING_WITH_LEN("\"\","));
+      buffer.append(STRING_WITH_LEN("\"0\","));
       continue;
     }
     else