fix: guard against undefined global state in createCacheHelper

[veeceey]

When using SWR with React Native's New Architecture (bridgeless mode), SWRGlobalState.get(cache) can return undefined during early renders before the provider has fully initialized. This causes a crash when createCacheHelper tries to destructure state[5] and state[6] from the undefined value.

This adds a simple null guard:

• The setter becomes a no-op when state is undefined (the update will happen once the provider initializes properly)
• The subscriber falls back to a function that returns a no-op unsubscribe handler

Tested locally - all existing unit and integration tests still pass.

Closes #2986

[veeceey]

Test results

All existing tests pass:

$ pnpm test -- --testPathPattern "test/unit" --no-coverage
Test Suites: 3 passed, 3 total
Tests:       9 passed, 9 total

$ pnpm test -- --testPathPattern "test/use-swr-cache" --no-coverage
Test Suites: 1 passed, 1 total
Tests:       20 passed, 20 total

$ pnpm test -- --testPathPattern "test/use-swr-remote-mutation" --no-coverage
Test Suites: 1 passed, 1 total
Tests:       33 passed, 33 total

Type checks also pass (tsc --noEmit via pre-commit hook).

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[veeceey]

hi, just wondering if this could get a look. let me know if anything needs updating!