sessionctx: supports set character_set_results = null

[imtbkcat]

What have you changed? (mandatory)

When tidb executes "set character_set_results = null", it will delete "character_set_results" variable because we set it to null. (see executor/set.go:79)

if value.IsNull() {
    	delete(sessionVars.Users, name)
}

This will delete "character_set_results". When we use show variable to show it, executor can not find it.
Then ShowExecutor will go to variable.SysVars to find its default value. (see executor/show.go:434)

varValue, ok := systemVars[varName]
if !ok {
    varValue = variable.SysVars[varName].Value
}

Originally, variable.SysVars["character_set_results"] is "latin1", so the return value of "show variables where variable_name='character_set_results'" is "latin1". But "character_set_results" in TiDB is always "utf8", set variable can not effect it.

This PR just fix compatibility with mysql when execute "set character_set_results = null".

What is the type of the changes? (mandatory)

• Improvement (non-breaking change which is an improvement to an existing feature)

How has this PR been tested? (mandatory)

unit test

After change:

mysql> set character_set_results = null;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables where variable_name='character_set_results';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| character_set_results |       |
+-----------------------+-------+
1 row in set (0.01 sec)

Does this PR affect documentation (docs/docs-cn) update? (mandatory)

No

Does this PR affect tidb-ansible update? (mandatory)

No

Does this PR need to be added to the release notes? (mandatory)

No

Refer to a related PR or issue link (optional)

Fix #7284

Benchmark result if necessary (optional)

Add a few positive/negative examples (optional)

[imtbkcat]

/run-all-tests

[coocood]

LGTM

[lysu]

In MySQL, it's very strange that will return NULL instead of "" in select @@ but return "" in show variable orz

mysql> select @@character_set_results ;
+-------------------------+
| @@character_set_results |
+-------------------------+
| utf8                    |
+-------------------------+
1 row in set (0.00 sec)

mysql> set character_set_results = NULL;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@character_set_results ;
+-------------------------+
| @@character_set_results |
+-------------------------+
| NULL                    |
+-------------------------+
1 row in set (0.00 sec)

mysql> show variables where variable_name='character_set_results';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| character_set_results |       |
+-----------------------+-------+
1 row in set (0.01 sec)

[jackysp]

I think system variables in MySQL have their own types.

[zz-jason]

LGTM