owner: fix that get owner opValue before set this value when the etcdClient is nil

[zimulala]

What problem does this PR solve?

Issue Number: close #45392

Problem Summary:
If do GetOwnerOpValue before set this value when the etcdClient is nil, we may get a panic.
The above scenario can occur because DDL's startDispatchLoop operation is asynchronous.

What is changed and how it works?

Make sure the mockOwnerOpValue is initialized before GetOwnerOpValue in bootstrap.

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

[tiprow]

Hi @zimulala. Thanks for your PR.

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[hawkingrei]

/ok-to-test

[zimulala]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Defined2014, xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-07-17 10:05:55.777170578 +0000 UTC m=+931247.547509291: ☑️ agreed by Defined2014.
• 2023-07-18 02:45:50.107032847 +0000 UTC m=+991241.877371560: ☑️ agreed by xhebox.

[hawkingrei]

/retest