workloadSettings optional field, but report an error

[allenhaozi]

Output of helm version:
version.BuildInfo{Version:"v3.0.3", GitCommit:"ac925eb7279f4a6955df663a0128044a8a6b7593", GitTreeState:"clean", GoVersion:"go1.13.6"}
Output of kubectl version:
Client Version: version.Info{Major:"1", Minor:"17", GitVersion:"v1.17.0", GitCommit:"70132b0f130acc0bed193d9ba59dd186f0e634cf", GitTreeState:"clean", BuildDate:"2019-12-07T21:20:10Z", GoVersion:"go1.13.4", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"17", GitVersion:"v1.17.0", GitCommit:"70132b0f130acc0bed193d9ba59dd186f0e634cf", GitTreeState:"clean", BuildDate:"2019-12-07T21:12:17Z", GoVersion:"go1.13.4", Compiler:"gc", Platform:"linux/amd64"}
Cloud Provider/Platform (AKS, GKE, Minikube etc.):
mini kube

Describe the bug
use oam-go-sdk create Component Schematic, If ComponentSchematic WorkloadSettings was not initialized, report an error

What you expected to happen:
No errors occur

logs:
"port","type":"string"}],"workloadSettings":null,"workloadType":"core.oam.dev/v1alpha1.Server"},"status":{}}
, Error("invalid type: null, expected a sequence", line: 1, column: 840)