Fix FileReader.result TypeScript type to include null

[Nedunchezhiyan-M]

Summary

The TypeScript declaration for FileReader.result is typed as string | ArrayBuffer, but the implementation (in Libraries/Blob/FileReader.js) returns ?ReaderResult which can be null. Per the WHATWG FileReader spec, result is null in the EMPTY and LOADING states.

This causes type mismatches when users check for null before using the result.

Changelog:

[GENERAL] [FIXED] - Fix FileReader.result TypeScript type to include null, matching the spec and implementation

Test Plan

1. Verify that FileReader.result typed as string | ArrayBuffer | null matches the WHATWG spec
2. Verify it matches the Flow source at Libraries/Blob/FileReader.js:178 which returns ?ReaderResult
3. TypeScript consumers can now correctly check for null before using the result

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