fix: skip preload when the key is falsy

[kwy404]

Root cause

preload() never checks the serialized key. With a falsy key (null, false, an empty array, or a key function that throws because its dependencies are not ready), serialize() returns '' and preload still calls fetcher(fnArg), so the fetcher runs with that falsy value (for example fetch('') requests the current page). The result is also stored in PRELOAD[''], so every later preload() with a falsy key returns that same stale promise, even with a different fetcher.

useSWR treats a falsy key as "not ready" and never calls the fetcher, so preload(user ? `/api/user/${user.id}` : null, fetcher) behaves differently from the hook it is meant to warm up.

Fix

Return early from preload when the serialized key is empty, the same way it already returns undefined on the server.

Test

Added should not call the fetcher when the key is falsy to test/use-swr-preload.test.tsx. It calls preload(null, fetcher) and preload(() => { throw ... }, fetcher) and expects the fetcher not to be called. Before the fix it fails with Received number of calls: 1 (called with null); after the fix the whole preload suite passes.