Fix FileReader.result type to include null

The TypeScript declaration for FileReader.result is typed as
string | ArrayBuffer, but the actual implementation returns null
when no read operation has completed or when the read was aborted.
This matches the WHATWG FileReader spec where result is null in
the EMPTY and LOADING states.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nedunchezhiyan-fin

packages/react-native/src/types/globals.d.ts

@@ -667,7 +667,7 @@ declare global {
       | ((this: FileReader, ev: ProgressEvent<FileReader>) => any)
       | null;
     readonly readyState: number;
-    readonly result: string | ArrayBuffer;
+    readonly result: string | ArrayBuffer | null;
     abort(): void;
     readAsArrayBuffer(blob: Blob): void;
     readAsDataURL(blob: Blob): void;