fix(image): skip 0-byte entries when initializing disk LRU cache

[hammadxcm]

What?

A single 0-byte (empty/corrupt) file in .next/cache/images/ permanently breaks image optimization for the rest of the process. This skips entries with a non-positive size when initializing the disk-LRU singleton.

Why?

initCacheEntries() reads such a file as a size: 0 entry. During init, getOrInitDiskLRU() replays entries via lru.set(entry.key, entry.size), but LRUCache.set() intentionally throws when size <= 0 (guard against unbounded growth). That throw rejects the singleton IIFE, and because _diskLRUPromise is only created when unset, the rejected promise is cached forever — every later ImageOptimizerCache get/set then awaits a rejected promise, so the image cache stays broken until the process restarts.

How?

In getOrInitDiskLRU(), guard the replay loop to skip entries with size <= 0. Done at the singleton layer so any source of a 0-size entry is handled, and it respects the existing LRUCache invariant rather than weakening it.

Verification

• pnpm --filter=next build
• pnpm --filter=next types
• node_modules/.bin/jest --config jest.config.js test/unit/image-optimizer/lru-disk-eviction.test.ts (8 passed, incl. new should skip a 0-byte entry instead of poisoning the singleton)
• pnpm prettier --write + eslint --fix on changed files (clean)

Fixes #93757