fix: Safeguard onSuccess callback to prevent errors when undefined

[mori-hisayuki]

Summary

Fixes an infinite loop caused by explicitly passing onSuccess: undefined in useSWR options.

Problem

When onSuccess: undefined is passed as a config property, it overrides the default noop function during config merging (mergeObjects uses object spread). This causes getConfig().onSuccess(...) to throw a TypeError, which is caught internally and triggers an error retry, resulting in an infinite request loop.

This commonly occurs when a custom hook accepts an optional onSuccess callback and passes it directly to useSWR:

const useMyFetch = ({ onSuccess }: { onSuccess?: (data: Data) => void }) => {
  return useSWR(key, fetcher, {
    onSuccess, // becomes undefined when the caller doesn't provide it
  });
};

Fix

Added optional chaining (?.) to the onSuccess invocation in use-swr.ts so that an undefined callback is safely skipped instead of throwing.

Related issue

Closes #4218

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