[Subtask][Flink]: improve unit test coverage in Flink module

[lklhdu]

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• I have searched in the issues and found no similar issues.

Description

we need to improve the unit test codes in module Flink and the goal is to increase unit test coverage to 90%.

Parent issue

#1598

Are you willing to submit PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[github-actions]

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