Auto merge of #24329 - tigleym:filereader_result, r=jdm

FileReader.read sets `result` to null before performing read operation

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As described in the spec, FileReader.read now sets its `result` to null after the FileReader's ready state is changed to "loading".
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #24288 (GitHub issue number if applicable)

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