[branch-2.10] Fix flaky test

[mattisonchao]

Motivation

Fix flaky tests of branch-2.10

Modifications

• testBatchMessageWithNullValue https://github.com/apache/pulsar/actions/runs/6075052806/job/16506780451?pr=21098
• testFetchPartitionedTopicMetadataWithCacheRefresh https://github.com/apache/pulsar/actions/runs/6075052814/job/16501956006?pr=21098

Verifying this change

• Make sure that the change passes the CI checks.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[poorbarcode]

The test LoadSimulationController also set the arg watch to null, could you fix it too?

[horizonzy]

LGTM