[INLONG-7769][DataProxy] Fix NPE when request Inlong Manager failed

[vernedeng]

Prepare a Pull Request

(Change the title refer to the following example)

• Title Example: [INLONG-XYZ][Component] Title of the pull request

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

• Fixes [Bug][DataProxy] NPE when request Inlong Manager failed #7769

Motivation

Explain here the context, and why you're making that change. What is the problem you're trying to solve?

Modifications

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Verifying this change

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• 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:

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

Documentation

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• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
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[luchunliang]

LGTM