fix: improve null safety in RequestHandlersV1

[VolodymyrBg]

• Replace unsafe null assertions (!!) with proper null checks
• Add explicit null validation for transaction hash parameter
• Add descriptive error messages for null cases
• Improve error handling in parameter parsing

These changes make the code more robust by preventing potential NullPointerExceptions and providing clearer error messages when invalid data is provided.

• I wrote new tests for my new core changes.
• I have successfully ran tests, style checker and build against my new changes locally.
• I have informed the team of any breaking changes if there are any.

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