fix: handle contract call transactions with no function args

Some contract call transactions have no function args.  The code
didn't handle that case.  On the current testnet, there are only
five such transactions.

src/rosetta-helpers.ts

@@ -171,7 +171,7 @@ function makeCallContractOperation(tx: BaseTx, index: number): RosettaOperation
         metadata: {
           contract_call_function_name: tx.contract_call_function_name,
           contract_call_function_args: bufferToHexPrefixString(
-            unwrapOptional(tx.contract_call_function_args, () => '')
+            tx.contract_call_function_args ? tx.contract_call_function_args : Buffer.from('')
           ),
           raw_result: tx.raw_result,
         },