fix: skip storage entries with missing preimage keys

[PinelliaC]

When GetKey (see https://github.com/ethereum/go-ethereum/blob/v1.15.11/trie/secure_trie.go#L235) is called, a missing preimage can cause the function to return a nil key. This, in turn, makes account.Storage persist an incorrect value.

[asdv23]

Great to see this optimized solution merged! To provide full context:
Root cause: Geth's init initialization without --cache.preimages flag caused incomplete preimage caching.
This became critical when:

1. Dumping state at specific block height for Reth's init-state initialization
2. Genesis alloc data wrote to stateDB without storing preimages
3. Contract execution later stored address preimages via SSTORE operations

Critical failure scenario:
When a storage key's preimage was:

• Stored due to contract execution, BUT
• Not originally cached from genesis alloc (because the specific storage slot wasn't accessed during genesis initialization)
  → The missing preimage caused the storage key to be misinterpreted as empty value during state dumping
  → Resulting in incorrect stateroot computation in init-state

My initial solution:
Fixed by manually constructing preimages for genesis alloc during dump: mantlenetworkio/op-geth@b609ebe, ensuring complete preimage coverage.

Optimization path:
After discussion with @PinelliaC, we concluded safely ignoring uncached storage-key preimages is valid because:

• Reth automatically populates genesis storage slots when importing genesis.json
• Thus omitted preimages during export are harmlessly reinitialized

This PR builds upon that collaborative insight.