privilege: fix select current_role() error (#15534)

[sre-bot]

cherry-pick #15534 to release-3.1

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What problem does this PR solve?

For user with no role, execute SET ROLE ALL, then SELECT CURRENT_ROLE()

mysql> set role all;
Query OK, 0 rows affected (0.00 sec)

mysql> select current_role();
ERROR 1105 (HY000): Missing session variable when eval builtin

Problem Summary:
This was caused by GetAllRole, which will return nil if user has no role.

What is changed and how it works?

let GetAllRole return a empty slice, rather than nil.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tidb-ansible:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Side effects

• None

Release note

[sre-bot]

/run-all-tests

[tiancaiamao]

LGTM

[jackysp]

LGTM

[jackysp]

/merge

[sre-bot]

/run-all-tests