Fix potential crash in FileReader.readAsArrayBuffer on malformed data URL

When reading as ArrayBuffer, the code splits the data URL on ","
and accesses index [1] without a fallback. If the native module
returns a string without a comma, this results in undefined being
passed to toByteArray(), causing a crash.

Added nullish coalescing to default to an empty string.

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

Author: nedunchezhiyan-fin

packages/react-native/Libraries/Blob/FileReader.js

@@ -86,7 +86,7 @@ class FileReader extends EventTarget {
           return;
         }
 
-        const base64 = text.split(',')[1];
+        const base64 = text.split(',')[1] ?? '';
         const typedArray = toByteArray(base64);
 
         this._result = typedArray.buffer;