privilege: fix select current_role() error

[imtbkcat]

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

[jackysp]

LGTM

[tiancaiamao]

LGTM

[tiancaiamao]

/merge

[sre-bot]

/run-all-tests

[codecov]

Codecov Report

Merging #15534 into master will increase coverage by 0.0299%.
The diff coverage is 86.9863%.

@@               Coverage Diff                @@
##             master     #15534        +/-   ##
================================================
+ Coverage   80.3979%   80.4278%   +0.0299%     
================================================
  Files           502        502                
  Lines        134149     134206        +57     
================================================
+ Hits         107853     107939        +86     
+ Misses        17822      17806        -16     
+ Partials       8474       8461        -13     

[sre-bot]

/run-all-tests

[sre-bot]

cherry pick to release-3.0 in PR #15569

[sre-bot]

cherry pick to release-3.1 in PR #15570