expression: unset the flen for string type builtin control

[tangenta]

What problem does this PR solve?

Issue Number: close #38844

Problem Summary: #38844 (comment)

What is changed and how it works?

For the prepare statements like ifnull(?,' '), we cannot infer the length of these expression. This PR unset the flen for the string return types.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Defined2014
• hawkingrei

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[Defined2014]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: bddb16c

[xiongjiwei]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 662262d

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #38849.

[sre-bot]

TiDB MergeCI notify

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-ddl-test	🟢 all 6 tests passed	29 min	Existing passed
idc-jenkins-ci/integration-cdc-test	🟢 all 38 tests passed	21 min	Existing passed
idc-jenkins-ci-tidb/integration-common-test	🟢 all 17 tests passed	14 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	8 min 56 sec	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	5 min 34 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	4 min 54 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	4 min 13 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	3 min 13 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	2 min 34 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed