branch-4.1: [fix](iceberg) execute empty static overwrite and stabilize info_schema timezone test

[suxiaogang223]

What problem does this PR solve?

This PR fixes two regression issues on branch-4.1.

1. Iceberg static partition overwrite was incorrectly treated as a no-op when the source query was optimized to an empty relation.
2. test_information_schema_timezone was flaky around information_schema.rowsets.

Solution

• Allow empty-source execution for Iceberg static partition overwrite so the transaction still reaches overwriteByRowFilter().
• Add a FE unit test covering empty-source static partition overwrite deleting only the matching partition.
• Stabilize the information_schema.rowsets regression case by waiting for a rowset result, ordering deterministically, and guarding nullable timestamps.

Test Result

• DISABLE_BUILD_UI=ON DISABLE_BUILD_HIVE_UDF=ON DISABLE_BE_JAVA_EXTENSIONS=ON ./build.sh --fe

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[suxiaogang223]

run buildall

[suxiaogang223]

run buildall

[hello-stephen]

FE UT Coverage Report

Increment line coverage 7.14% (2/28) 🎉
Increment coverage report
Complete coverage report

[yiguolei]

skip buildall

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.