Describe the bug
System variables for MariaDB have either Session, Global, or both scopes.
However, HeidiSQL's System Variables table does not show null for variables that do not exist in that respective scope.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://mariadb.com/kb/en/show-variables/
2. Per the second example, in Heidi, run the following:

SELECT VARIABLE_NAME, SESSION_VALUE, GLOBAL_VALUE FROM
  INFORMATION_SCHEMA.SYSTEM_VARIABLES WHERE
  VARIABLE_NAME LIKE 'general_log_file';

3. The result is:

+---------------------------+---------------+--------------+
| VARIABLE_NAME             | SESSION_VALUE | GLOBAL_VALUE |
+---------------------------+---------------+--------------+
| GENERAL_LOG_FILE          | (NULL)        | /path/log    |
+---------------------------+---------------+--------------+

4. Compare with the System Variables table (select a database, Variables tab), 'general_log_file' entry.

Expected behavior
Expecting to see a null indication in the Session column.

Comments
The MariaDB page says at the bottom:

[If a] variable only has a global scope, the global value is returned even when specifying [SHOW] SESSION.
When having HeidiSQL show this table, these queries are issued:

SHOW VARIABLES;
SHOW GLOBAL VARIABLES;

Suggest not using SHOW but rather SELECT.

(Of course, this supposes whether INFORMATION_SCHEMA.SYSTEM_VARIABLES is universal.)

Environment:

• OS: Win 7
• HeidiSQL version 11.1.0.6116
• 10.4.12-MariaDB-1:10.4.12+maria~bionic-log