Improve handling of mirror renw

[dveeden]

What problem does this PR solve?

GetComponentManifest() can return a nil manifest if it returns an error. newMirrorRenewCmd() ignores expiration errors when calling GetComponentManifest(). It should make sure that the manifest is not nil when calling RenewManifest().

ref #2398

What is changed and how it works?

Check List

Tests

• Manual test (add detailed scripts or steps below)

Release notes:

Bugfix: The `tiup mirror renew` command could segfault on expired manifests.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 55.93%. Comparing base (6f01692) to head (d11e769).

:exclamation: Current head d11e769 differs from pull request most recent head 5ab50e4. Consider uploading reports for the commit 5ab50e4 to get more accurate results

Files	Patch %	Lines
cmd/mirror.go	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2400 +/- ## ========================================== - Coverage 55.95% 55.93% -0.02% ========================================== Files 329 329 Lines 35221 35224 +3 ========================================== - Hits 19706 19702 -4 - Misses 13143 13149 +6 - Partials 2372 2373 +1 ``` | [Flag](https://app.codecov.io/gh/pingcap/tiup/pull/2400/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | Coverage Δ | | |---|---|---| | [tiup](https://app.codecov.io/gh/pingcap/tiup/pull/2400/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `33.58% <0.00%> (-0.06%)` | :arrow_down: | | [unittest](https://app.codecov.io/gh/pingcap/tiup/pull/2400/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `22.15% <0.00%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap#carryforward-flags-in-the-pull-request-comment) to find out more.

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[kaaaaaaang]

/lgtm

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-04-09 12:20:06.272743816 +0000 UTC m=+964867.800284360: :ballot_box_with_check: agreed by kaaaaaaang.