[flink] Preserve offsets for empty tiering buckets

[hutiefang76]

What is changed

Fixes #3413.

This change keeps lake writes limited to non-empty bucket write results, but collects committed Fluss log offsets and max tiered timestamps from all completed bucket write results in a mixed commit round. That preserves offsets for buckets that only advanced through an empty materialized batch while another bucket produced lake data in the same commit.

The existing empty-write-result test now asserts that the empty bucket offset is included in the committed lake snapshot metadata.

Why

Before this change, TieringCommitOperator built the logEndOffsets map from only non-empty write results. In a mixed commit, empty-batch buckets with writeResult() == null were filtered out, so their completed log offsets were not committed even though the commit round had lake data to publish.

Tests

• Red/green verified TieringCommitOperatorTest#testCommitMeetsEmptyWriteResult; before the production change it failed because bucket 0 offset was missing from the committed snapshot.
• mvn -s <empty-settings> -pl fluss-flink/fluss-flink-common -am -Dtest=TieringCommitOperatorTest#testCommitMeetsEmptyWriteResult -Dsurefire.failIfNoSpecifiedTests=false test
• mvn -s <empty-settings> -pl fluss-flink/fluss-flink-common -am -Dtest=TieringCommitOperatorTest -Dsurefire.failIfNoSpecifiedTests=false test
• git diff --check

I used an empty temporary Maven settings file locally to avoid a private mirror configured in my user Maven settings.

AI assistance

This PR was prepared with assistance from OpenAI Codex. I reviewed the change and verified the tests above.