support nil wasmplugin config in ingress-level (#852)

pkg/ingress/config/ingress_config.go

@@ -819,7 +819,9 @@ func (m *IngressConfig) convertIstioWasmPlugin(obj *higressext.WasmPlugin) (*ext
 				continue
 			}
 			if rule.Config == nil {
-				return nil, errors.New("invalid rule has no config")
+				rule.Config = &types.Struct{
+					Fields: map[string]*types.Value{},
+				}
 			}
 			v := &types.Value_StructValue{
 				StructValue: rule.Config,