[flink] Fix flaky test duplicate nullable PK IT

[ArnavBalyan]

Purpose

• The test inserts two null pks in one insert and asserts the second one wins.
• Sequence numbers are assigned on arrival and deduplication keeps the highest, so the surviving row varies depending on the arrival order. (18 of 39 runs failed)
• Fix the assertion to be compatible with the deduplication. All runs pass post fix.

Tests

• UT fix

[ArnavBalyan]

#9118 recently merged fixes similar assertion but was not checking that full row survives deduplication. This PR checks stronger full row coverage.

[ArnavBalyan]

CI failure is transient

[JingsongLi]

+1