KAFKA-5171 : TC should not accept empty string transactional id

[umesh9794]

This is an initial PR. Changed the unit tests accordingly as per the expectation from TC.

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[umesh9794]

@guozhangwang please review this PR and let me know if that serves the purpose. Can we also refactor TransactionCoordinator.handleInitPid() to leverage the existing validateTransactionalId() method?

[guozhangwang]

@umesh9794 thank you for the PR. I think the purpose is to accept null transactionalId but not empty string, while your PR changes both? Also there are a couple of PRs ongoing that may change TransactionalCoordinator class as well, so your PR may need to be rebased after those commits, just a heads up :)

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[umesh9794]

Ah..sorry I missed that null should be accepted, thanks for the review @guozhangwang. I have updated the PR. Sure will wait to rebase this PR after those commits and thanks again for telling this in advance :)

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[umesh9794]

@guozhangwang should I rebase it now or some more changes are about to be pushed?

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[guozhangwang]

@guozhangwang should I rebase it now or some more changes are about to be pushed?

The major changes have been pushed, feel free to rebase now.

[umesh9794]

@guozhangwang . I rebased the changes and created a new PR because I got messed with many conflicts in this PR. Please review that updated PR and let me know your comments.