[Fix]partial-update still keeps row when the last message is -U only

[sandyfog]

Purpose

In streaming pipelines, a common Flink SQL pattern is to filter rows before they reach the Paimon sink:

INSERT INTO paimon_table
SELECT * FROM upstream WHERE delete = 0;

When a row flips from delete = 0 to delete = 1, Flink first emits an UPDATE_BEFORE (-U) for the previously qualified row.
The succeeding UPDATE_AFTER (+U) is dropped by the filter, so only the -U record arrives at Paimon.
The current PartialUpdateMergeFunction does not treat this lone -U as a deletion.
Consequently, the stale row remains in the table, breaking correctness.

Linked issue: close #6862

Tests

API and Format

Documentation

[sandyfog]

@JingsongLi PTAL, thanks!