fix: handle localStorage null (approach B: sentinel value)

[astoilkov]

Summary

Fixes #80 — Firefox with dom.storage.enabled: false sets localStorage to null, causing the hook to break.

Approach: Fix initial ref sentinel value

The root cause: storageItem.current.string is initialized to null, and when localStorage.getItem() fails, goodTry returns undefined which gets coerced to null. Since both values are null, the comparison string !== storageItem.current.string is false, skipping the parsing branch — the value stays undefined instead of defaultValue.

Changes:

1. Sentinel value: Changed storageItem ref initial string from null to undefined, so first-render comparison null !== undefined is true — the parsing branch always runs
2. Default-value writing: Replaced goodTry with try-catch that falls back to inMemoryData.set(key, defaultValue) when localStorage is unavailable
3. removeItem: Reset storageItem.current.string to undefined so getSnapshot re-parses correctly after removal when localStorage is unavailable

Why this approach:

• Fixes the root cause — the null-null comparison that skips parsing
• The undefined sentinel is semantically correct: "not yet read from localStorage"
• The try-catch fallback in default-value writing ensures inMemoryData is populated, making the existing isPersistent logic work correctly
• The removeItem sentinel reset handles the edge case of removing + re-defaulting when localStorage is unavailable

Alternative approach

See PR #82 for approach A (detect unavailability) — a different way to solve the same issue.

Next.js test projects

To test in Firefox with dom.storage.enabled: false in about:config:

• main branch (shows the bug): https://localstorage-null-test-main-9slssprfd-astoilkov1s-projects.vercel.app
• this PR (approach B fix): https://localstorage-null-test-approach-2ogr0txkp-astoilkov1s-projects.vercel.app

Test plan

• Added 7 tests simulating localStorage being null via vi.spyOn(window, 'localStorage', 'get').mockReturnValue(null)
• Tests cover: defaultValue, isPersistent, setValue, setValue with callback, removeItem, no defaultValue