ctl: fix https client panic (#8239)

[ti-chi-bot]

This is an automated cherry-pick of #8239

What problem does this PR solve?

Issue Number: Close #8237, Ref #7300

What is changed and how does it work?

fix panic when call pd-ctl cluster with tls

Check List

Tests

• Unit test

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

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

Release note

None.

[ti-chi-bot]

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[okJiang]

/cherry-pick-invite

[ti-chi-bot]

@okJiang Please accept the invitation then you can push to the cherry-pick pull requests.
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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HuSharp, rleungx

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• OWNERS [HuSharp,rleungx]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-29 09:31:55.562319655 +0000 UTC m=+175431.842367724: ☑️ agreed by HuSharp.
• 2024-07-29 09:33:05.436109811 +0000 UTC m=+175501.716157880: ☑️ agreed by rleungx.

[ti-chi-bot]

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