MDEV-32906: The SQL error plugin prints (null) as database if the mariadb

client is not using any database to execute the SQL.

Analysis:
When there is no database, the database string is NULL so (null) gets
printed.
Fix:
Print NULL instead of (null) because when there is no database SELECT
DATABASE() return NULL. SO NULL is more appropriate choice.

mysql-test/suite/plugins/r/mdev_27087.result

-show variables like 'sql_error_log%';
-Variable_name	Value
-sql_error_log_filename	sql_errors.log
-sql_error_log_rate	1
-sql_error_log_rotate	OFF
-sql_error_log_rotations	9
-sql_error_log_size_limit	1000000
-sql_error_log_with_db_and_thread_info	ON
-set global sql_error_log_rate=1;
-select * from t_doesnt_exist;
-ERROR 42S02: Table 'test.t_doesnt_exist' doesn't exist
-THREAD_ID DATABASE_NAME TIME HOSTNAME ERROR 1146: Table 'test.t_doesnt_exist' doesn't exist : select * from t_doesnt_exist 

mysql-test/suite/plugins/r/sql_error_log_withdbinfo.result

+show variables like 'sql_error_log%';
+Variable_name	Value
+sql_error_log_filename	sql_errors.log
+sql_error_log_rate	1
+sql_error_log_rotate	OFF
+sql_error_log_rotations	9
+sql_error_log_size_limit	1000000
+sql_error_log_with_db_and_thread_info	ON
+set global sql_error_log_rate=1;
+# Trying to set the variable at runtime
+SET sql_error_log_with_db_and_thread_info=OFF;
+ERROR HY000: Variable 'sql_error_log_with_db_and_thread_info' is a read only variable
+#
+# Using test database from mtr
+#
+DROP DATABASE db;
+ERROR HY000: Can't drop database 'db'; database doesn't exist
+#
+# Using no database at all
+#
+DROP DATABASE test;
+DROP DATABASE db;
+ERROR HY000: Can't drop database 'db'; database doesn't exist
+#
+# Using database with name `NULL`
+#
+CREATE DATABASE `NULL`;
+USE `NULL`;
+DROP DATABASE db;
+ERROR HY000: Can't drop database 'db'; database doesn't exist
+THREAD_ID `test` TIME HOSTNAME ERROR 1238: Variable 'sql_error_log_with_db_and_thread_info' is a read only variable : SET sql_error_log_with_db_and_thread_info=OFF 
+THREAD_ID `test` TIME HOSTNAME ERROR 1008: Can't drop database 'db'; database doesn't exist : DROP DATABASE db 
+THREAD_ID NULL TIME HOSTNAME ERROR 1008: Can't drop database 'db'; database doesn't exist : DROP DATABASE db 
+THREAD_ID `NULL` TIME HOSTNAME ERROR 1008: Can't drop database 'db'; database doesn't exist : DROP DATABASE db 
+DROP DATABASE `NULL`;
+# Reset
+CREATE DATABASE test;

mysql-test/suite/plugins/t/mdev_27087.test

---source include/not_embedded.inc
-
-if (!$SQL_ERRLOG_SO) {
-  skip No SQL_ERROR_LOG plugin;
-}
-
-show variables like 'sql_error_log%';
-set global sql_error_log_rate=1;
---error ER_NO_SUCH_TABLE
-select * from t_doesnt_exist;
-
-let $MYSQLD_DATADIR= `SELECT @@datadir`;
---let SEARCH_FILE= $MYSQLD_DATADIR/sql_errors.log
---let LINES_TO_READ=1
---replace_regex /[1-9]* test [1-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9] [ 0-9][0-9]:[0-9][0-9]:[0-9][0-9] [^E]*/THREAD_ID DATABASE_NAME TIME HOSTNAME /
---source include/read_head.inc
-
-remove_file $MYSQLD_DATADIR/sql_errors.log;

mysql-test/suite/plugins/t/mdev_27087.opt → mysql-test/suite/plugins/t/sql_error_log_withdbinfo.opt

---plugin-load-add=$SQL_ERRLOG_SO --sql-error-log-with-db-and-thread-info=1
+--plugin-load-add=$SQL_ERRLOG_SO --sql-error-log-with-db-and-thread-info=1 --lower_case_table_names=2

mysql-test/suite/plugins/t/sql_error_log_withdbinfo.test

+--source include/not_embedded.inc
+
+if (!$SQL_ERRLOG_SO) {
+  skip No SQL_ERROR_LOG plugin;
+}
+
+show variables like 'sql_error_log%';
+set global sql_error_log_rate=1;
+
+let $MYSQLD_DATADIR= `SELECT @@datadir`;
+
+--echo # Trying to set the variable at runtime
+
+--error ER_INCORRECT_GLOBAL_LOCAL_VAR
+SET sql_error_log_with_db_and_thread_info=OFF;
+
+--echo #
+--echo # Using test database from mtr
+--echo #
+
+--error ER_DB_DROP_EXISTS
+DROP DATABASE db;
+
+--echo #
+--echo # Using no database at all
+--echo #
+
+DROP DATABASE test;
+--error ER_DB_DROP_EXISTS
+DROP DATABASE db;
+
+--echo #
+--echo # Using database with name `NULL`
+--echo #
+CREATE DATABASE `NULL`;
+USE `NULL`;
+--error ER_DB_DROP_EXISTS
+DROP DATABASE db;
+
+
+--let SEARCH_FILE= $MYSQLD_DATADIR/sql_errors.log
+--let LINES_TO_READ=4
+--replace_regex /[1-9]* `NULL` [1-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9] [ 0-9][0-9]:[0-9][0-9]:[0-9][0-9] [^E]*/THREAD_ID `NULL` TIME HOSTNAME /  /[1-9]* `test` [1-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9] [ 0-9][0-9]:[0-9][0-9]:[0-9][0-9] [^E]*/THREAD_ID `test` TIME HOSTNAME /  /[1-9]* NULL [1-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9] [ 0-9][0-9]:[0-9][0-9]:[0-9][0-9] [^E]*/THREAD_ID NULL TIME HOSTNAME /
+--source include/read_head.inc
+
+DROP DATABASE `NULL`;
+
+--echo # Reset
+CREATE DATABASE test;

plugin/sql_errlog/sql_errlog.c

@@ -15,6 +15,7 @@
 
 #include <mysql/plugin_audit.h>
 #include <stdio.h>
+#include <string.h>
 #include <time.h>
 #include <mysql/service_logger.h>
 
@@ -91,6 +92,7 @@ static void log_sql_errors(MYSQL_THD thd __attribute__((unused)),
 {
   const struct mysql_event_general *event =
          (const struct mysql_event_general*)ev;
+
   if (rate &&
       event->event_subclass == MYSQL_AUDIT_GENERAL_ERROR)
   {
@@ -103,14 +105,26 @@ static void log_sql_errors(MYSQL_THD thd __attribute__((unused)),
       (void) localtime_r(&event_time, &t);
       if (with_db_and_thread_info)
       {
-        logger_printf(logfile, "%llu %s %04d-%02d-%02d %2d:%02d:%02d "
+        if (event->database.str)
+        {
+          logger_printf(logfile, "%llu %`s %04d-%02d-%02d %2d:%02d:%02d "
                       "%s ERROR %d: %s : %s \n",
               event->general_thread_id, event->database.str, t.tm_year + 1900,
-              t.tm_mon + 1, t.tm_mday, t.tm_hour, t.tm_min, t.tm_sec,
-              event->general_user, event->general_error_code,
+              t.tm_mon + 1, t.tm_mday, t.tm_hour, t.tm_min,
+              t.tm_sec, event->general_user, event->general_error_code,
               event->general_command, event->general_query);
         }
         else
+        {
+          logger_printf(logfile, "%llu %s %04d-%02d-%02d %2d:%02d:%02d "
+                      "%s ERROR %d: %s : %s \n",
+              event->general_thread_id, "NULL", t.tm_year + 1900,
+              t.tm_mon + 1, t.tm_mday, t.tm_hour, t.tm_min,
+              t.tm_sec, event->general_user, event->general_error_code,
+              event->general_command, event->general_query);
+        }
+      }
+      else
       {
         logger_printf(logfile, "%04d-%02d-%02d %2d:%02d:%02d "
                       "%s ERROR %d: %s : %s\n",