[fix][fn] Fix ProducerConfig cannot update error

[jiangpengcheng]

Fixes #21036

Main Issue: #xyz

PIP: #xyz

Motivation

while merging existing config with new config for updating, it doesn't check whether the new config has set the ProducerConfig, which makes the merged config always use the ProducerConfig from the existing config

Modifications

set the producerConfig when updating functions/sources if the ProducerConfig is specified

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

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

(or)

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

(or)

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

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

Matching PR in forked repository

PR in forked repository: jiangpengcheng#15

[Technoboy-]

Better to add a test

[mattisonchao]

Would you mind adding a unit test for this change? :)

[jiangpengcheng]

@Technoboy- @mattisonchao
tests added