[TS migration] Update MERGE method value type

[VickyStash]

Details

This PR updates the MERGE method value type. During TS migration of IOU.js file in E/App there were found several places where null was passed as a value. After testing, it turned out it works as expected, so this PR updates value typing to support null as well.

Examples:
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Related Issues

Expensify/App#34092

Automated Tests

N/A

Manual Tests

N/A

Author Checklist

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