Simply use `select_value` in `show_variable`

`SELECT @@name` statement returns only single row or `StatementInvalid`.

```
root@localhost [activerecord_unittest] > SELECT @@version;
+-----------+
| @@version |
+-----------+
| 5.7.9-log |
+-----------+
1 row in set (0.00 sec)

root@localhost [activerecord_unittest] > SELECT @@unknown_variable;
ERROR 1193 (HY000): Unknown system variable 'missing_variable'
```

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -585,8 +585,7 @@ def type_to_sql(type, limit = nil, precision = nil, scale = nil, unsigned = nil)
 
       # SHOW VARIABLES LIKE 'name'
       def show_variable(name)
-        variables = select_all("select @@#{name} as 'Value'", 'SCHEMA')
-        variables.first['Value'] unless variables.empty?
+        select_value("SELECT @@#{name}", 'SCHEMA')
       rescue ActiveRecord::StatementInvalid
         nil
       end