pathdb: infer stateIdent type from storage hash

[weiihann]

This PR simplifies the stateIdent struct by removing the explicit account boolean field and instead inferring account status by checking if storageHash is empty. Advantages:

• Self-documenting data model: Derive status from the data itself rather than relying on a separate flag
• Eliminates potential inconsistencies: Prevents situations where the boolean flag could be out of sync with the hash value
• Cleaner API: Constructors now enforce the invariant that storage identifiers must have non-empty hashes

While comparing a 32-byte hash is slightly less performant than a boolean check, this design improves code clarity and reduces the chance of bugs where the boolean and hash values contradict each other.