Fix global state root when class root is empty

[Mohiiit]

Summary

• return the contract state root directly when the class commitment root is zero
• align the Cairo implementation with the documented backward-compatibility rule

Why

When the class tree is empty, the global state root should equal the contract state root. The previous Cairo code only short-circuited when both roots were zero, which caused block 0 to hash (GLOBAL_STATE_VERSION, contract_state_root, 0) and produce the wrong final root in SNOS.

Validation

• reproduced the mismatch from SNOS against block 0
• verified that before OS execution the RPC root already matched the contract-state root and the class root was zero
• reran SNOS against this patched sequencer checkout and confirmed the final OS root matched the RPC root: 0x3c538d437670f4c6f72dd799f215a007720ec7d19bc64195c96399145d8746f