fix(code): make CfnResource#_toCloudFormation null-safe

The `CfnResource#_toCloudFormation` method creates a `PostResolveToken`
with a post-processor that was not ready to handle the absence of
`Properties` on the resolved value. It is however possible that
`Properties` are missing when an object is created with the default
configuration (e.g: by `new sqs.CfnQueue(this, 'Queue');`).

This change makes the post-processor function correctly handle
`undefined` in this case.

Related #3093

packages/@aws-cdk/core/lib/cfn-resource.ts

@@ -230,7 +230,7 @@ export class CfnResource extends CfnRefElement {
             Metadata: ignoreEmpty(this.cfnOptions.metadata),
             Condition: this.cfnOptions.condition && this.cfnOptions.condition.logicalId
           }, props => {
-            props.Properties = this.renderProperties(props.Properties);
+            props.Properties = props.Properties && this.renderProperties(props.Properties);
             return deepMerge(props, this.rawOverrides);
           })
         }