BE: Messages: Support sending null headers

[AkashDeepSinghJassal]

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

What changes did you make? (Give an overview)
Fix #620
Allow null values in headers when sending messages to the Kafka topic.
There is a missing null check while converting String to byte array, so I have added a null check to avoid converting to bytes if the value String is null.
Also I wrote a test case that replicated the given scenario

 java.lang.NullPointerException: Cannot invoke "String.getBytes()" because "v" is null	at io.kafbat.ui.serdes.ProducerRecordCreator.lambda$createHeaders$0(ProducerRecordCreator.java:34) ...

After fixing the issue, the test case ran successfully.

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

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

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[github-actions]

Hi AkashDeepSinghJassal! 👋

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.

[AkashDeepSinghJassal]

How to resolve failed checks @Haarolean, please let me know.

[germanosin]

Nice catch! Thanks!

[Haarolean]

@AkashDeepSinghJassal thanks for your first contribution to kafbat ui!