[relay-runtime] return undefined from readInlineFragment

[robrichard]

Same change was done to useFragment here: https://github.com/DefinitelyTyped/DefinitelyTyped/pull/67136/files

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[typescript-bot]

@robrichard Thank you for submitting this PR!

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[typescript-bot]

🔔 @alloy @maraisr @morrys @levibuzolic — please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.