[INLONG-5637][Sort] Fix kafka load node npe error

[yunqingmoswu]

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Title: [INLONG-5637][Sort] Fix kafka load node npe error

(The following XYZ should be replaced by the actual GitHub Issue number)

Fixes #5637

Motivation

Fix the npe error of kafka load node

Modifications

Update the logic of sendOutMetrics and sendDirtyMetrics in FlinkKafkaProducer.

Verifying this change

(Please pick either of the following options)

• This change is a trivial rework/code cleanup without any test coverage.

• This change is already covered by existing tests, such as:
  (please describe tests)

• This change added tests and can be verified as follows:

  (example:)

  • Added integration tests for end-to-end deployment with large payloads (10MB)
  • Extended integration test for recovery after broker failure

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a follow-up issue for adding the documentation