security: Allow empty security config for disabling tls (#9234)

[ti-chi-bot]

This is an automated cherry-pick of #9234

What problem does this PR solve?

Issue Number: close #9235

Problem Summary:

TiFlash fails to start with following configs

        [security]
          ca_path = ""
          cert_path = ""
          key_path = ""

What is changed and how it works?

If any of the security.ca_path/security.cert_path/security.key_path is empty, treat it as missing.
And if all of the three config are not exist or empty strings, TLS is disabled in tiflash.

security: allow empty `security.ca_path`/`security.cert_path`/`security.key_path`

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

Fix a bug that empty SSL config strings will enable TLS for tiflash

[JaySon-Huang]

/run-all-tests

[windtalker]

LGTM

[JaySon-Huang]

/run-integration-test

[JaySon-Huang]

/run-all-tests

[JaySon-Huang]

/run-integration-test

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JaySon-Huang, windtalker

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files:

• OWNERS [JaySon-Huang,windtalker]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-18 01:09:56.076001551 +0000 UTC m=+489018.066943020: ☑️ agreed by windtalker.
• 2024-07-19 02:58:02.66558038 +0000 UTC m=+581904.656521850: ☑️ agreed by JaySon-Huang.