client/http: fix the panic when merging empty RegionsInfo

[JmPotato]

What problem does this PR solve?

Issue Number: ref #7300, pingcap/tidb#52013.

What is changed and how does it work?

Check if the source and target `*RegionsInfo` are nil before merging.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• HuSharp

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[JmPotato]

/merge

[ti-chi-bot]

@JmPotato: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: c7b8643