Support PostgreSQL do nothing conflict action

[TherChenYang]

Fixes #31046.

Changes proposed in this pull request:

• Support PostgreSQL do nothing conflict action

This submission resolves the NullPointerException of the following SQL.

INSERT INTO sj_event ( event_id, event_type, event_status, content, create_time, update_time )
VALUES (?,?,?,?,?,?) ON CONFLICT ( event_id ) DO NOTHING

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[strongduanmu]

Good job, merged. @TherChenYang Please check if openGauss has the same problem?

[TherChenYang]

@strongduanmu Okay, I will check it

[TherChenYang]

@strongduanmu I have completed the check, and there are no same issues in openGauss. The upfront null check has been done in the code.