fix: contracts with no private / public functions should not fail to …

…generate a contract artifact (#11744)

If there were no public / private functions, the abi structs for
functions is undefined. In the previous case, with any private / public
function, unconstrained functions would just not exist in the
abistructs, and returnTypes would stay an empty array, but if there were
no private / public functions, abiStructs would be undefined and we
would throw an error. This fixes that.

yarn-project/types/src/abi/contract_artifact.ts

@@ -128,8 +128,8 @@ function generateFunctionArtifact(
   }
 
   let returnTypes: AbiType[] = [];
-  if (functionType === FunctionType.UNCONSTRAINED && fn.abi.return_type) {
-    returnTypes = [fn.abi.return_type.abi_type];
+  if (functionType === FunctionType.UNCONSTRAINED) {
+    returnTypes = fn.abi.return_type ? [fn.abi.return_type.abi_type] : returnTypes;
   } else {
     const pathToFind = `${contract.name}::${fn.name}_abi`;
     const abiStructs: AbiType[] = contract.outputs.structs['functions'];