fix optinal albi

Author: jbcaron

starknet-core/src/types/contract/legacy.rs

@@ -31,6 +31,7 @@ const API_VERSION: Felt = Felt::ZERO;
 #[cfg_attr(feature = "no_unknown_fields", serde(deny_unknown_fields))]
 pub struct LegacyContractClass {
     /// Contract ABI.
+    #[serde(default)]
     pub abi: Vec<RawLegacyAbiEntry>,
     /// Contract entrypoints.
     pub entry_points_by_type: RawLegacyEntryPoints,