session: invalid pending txn if it's a empty stmt

[lysu]

What problem does this PR solve?

Issue Number: close #15716 in 3.0

#11981 has fixed it in 4.0

Problem Summary:

execute ‘-- T1’ will be parsed as empty stmt and start a txn but without closing it after empty stmt finished

What is changed and how it works?

What's Changed:

close txn after empty stmt returned

How it Works:

close txn after empty stmt returned

Related changes

• n/a

Check List

Tests

• Integration test(WIP)
• Manual test (add detailed scripts or steps below)

Side effects

• n/a

Release note

invalid pending txn if it's a empty stmt

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This change is 

[lysu]

we have another chosen is cherry-pick #11981 - -

@coocood @tiancaiamao @jackysp