session, ddl: fix the nil point in TxnInfo (#44486)

[ti-chi-bot]

This is an automated cherry-pick of #44486

What problem does this PR solve?

Issue Number: close #43829

Problem Summary:

processInfo := s.ShowProcess()
txnInfo.ConnectionID = processInfo.ID   // panic is this line.

What is changed and how it works?

• Check if the processInfo is nil.
• Delete the duplicate logic of getting processInfoID.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code
  Hard to test. In UT we use the MockSessionManager, so can't to test it.

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

[ti-chi-bot]

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