[EventPolling] Fixed issues in throwing an unhandled exception when payload is empty

[Ashi1993]

[EventPolling] Fixed issues in throwing an unhandled exception when payload is empty

This PR contains the fix for issue in throwing an unhandled exception when payload is empty

Issue link: https://github.com/wso2-enterprise/financial-open-banking/issues/8320

Doc Issue: Optional, link issue from documentation repository

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Development Checklist

1. Build complete solution with pull request in place.
2. Ran checkstyle plugin with pull request in place.
3. Ran Findbugs plugin with pull request in place.
4. Ran FindSecurityBugs plugin and verified report.
5. Formatted code according to WSO2 code style.
6. Have you verified the PR doesn't commit any keys, passwords, tokens, usernames, or other secrets?
7. Migration scripts written (if applicable).
8. Have you followed secure coding standards in WSO2 Secure Engineering Guidelines?

Testing Checklist

1. Written unit tests.
2. Verified tests in multiple database environments (if applicable).
3. Tested with BI enabled (if applicable).