fix: preserve null values in json-avro conversion

[menezestrigueiros]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)

• Updated JSON conversion for Avro record fields in JsonAvroConversion.java:207-216.
• Before: null-valued fields were conditionally skipped depending on showNullValues.
• Now: all record fields are always emitted; when a field value is null, it is explicitly written as JSON null (MAPPER.nullNode()).
• Preserves schema-defined fields in output and avoids dropping null properties during Avro → JSON conversion.

Is there anything you'd like reviewers to focus on?

• Confirm the intended behavior is to always preserve field presence for Avro record outputs, even when values are null.
• Validate this aligns with existing Kafka UI expectations for message rendering and Schema Registry-backed payloads.
• Check for any downstream consumers/tests that relied on null fields being omitted.
• Sanity-check that non-null conversion behavior is unchanged.

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• No need to
• Manually (please, describe, if necessary)
• Unit checks
• Integration checks
• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

Check out Contributing and Code of Conduct

A picture of a cute animal (not mandatory but encouraged)
This is my cat, Mel :)

[github-actions]

Hi menezestrigueiros! 👋

Welcome, and thank you for opening your first PR in the repo!

Please wait for triaging by our maintainers.

Please take a look at our contributing guide.

[joshuaNathaniel]

@menezestrigueiros You may want to revisit #1639