br: fix br debug encode panic (#40880)

[ti-chi-bot]

This is an automated cherry-pick of #40880

Signed-off-by: zhanggaoming gaoming.zhang@pingcap.com

What problem does this PR solve?

Issue Number: close #40878

Problem Summary:
The goroutinue will panic when executing br debug encode.

What is changed and how it works?

New backuppb.Schema to avoid nil pointer dereference

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

Manual test

• br backup full
• br debug decode
• br debug encode
• success

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fixed a bug that caused: execute br debug encode panic.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• MoCuishle28
• fengou1

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[YuJuncen]

/merge

[ti-chi-bot]

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

Commit hash: a0df92e

[MoCuishle28]

/retest

[joccau]

/merge