cdk.Output should validate that it is non-empty

[mahe-work]

When trying to create a VpcNetwork without VpnGateway, VpcNetworks import props cannot be exported.

cdk deploy output:

cdk  deploy 
vpc-stack: deploying...
vpc-stack: creating CloudFormation changeset...

 ❌  vpc-stack failed: ValidationError: Template format error: Every Outputs member must contain a Value object
Template format error: Every Outputs member must contain a Value object

vpc.ts:

import cdk = require("@aws-cdk/cdk");
import ec2 = require("@aws-cdk/aws-ec2");

export interface VPCStackProps extends cdk.StackProps {
    cidr: string,
}

export class VPCStack extends cdk.Stack {
    public readonly exports: ec2.VpcNetworkImportProps;

    constructor(scope: cdk.App, id: string, props: VPCStackProps) {
        super(scope, id)
        const vpc = new ec2.VpcNetwork(this, "vpc", {
            cidr: props.cidr,
            subnetConfiguration: [
                {
                    cidrMask: 26,
                    name: "Public",
                    subnetType: ec2.SubnetType.Public
                },
                {
                    cidrMask: 26,
                    name: "Private",
                    subnetType: ec2.SubnetType.Private
                },
                {
                    cidrMask: 27,
                    name: "DB",
                    subnetType: ec2.SubnetType.Isolated
                },
            ],
            vpnGateway: false
        })

        this.exports = vpc.export();
    }
}

cfn template Outputs section from cdk synth:

Outputs:
  vpcPublicSubnetIDs7CDCF73A:
    Value:
      Fn::Join:
        - ","
        - - Ref: vpcPublicSubnet1Subnet2E65531E
          - Ref: vpcPublicSubnet2Subnet009B674F
          - Ref: vpcPublicSubnet3Subnet11B92D7C
    Export:
      Name: vpc-stack:vpcPublicSubnetIDs7CDCF73A
  vpcPrivateSubnetIDs121CC6B5:
    Value:
      Fn::Join:
        - ","
        - - Ref: vpcPrivateSubnet1Subnet934893E8
          - Ref: vpcPrivateSubnet2Subnet7031C2BA
          - Ref: vpcPrivateSubnet3Subnet985AC459
    Export:
      Name: vpc-stack:vpcPrivateSubnetIDs121CC6B5
  vpcIsolatedSubnetIDs67369DC2:
    Value:
      Fn::Join:
        - ","
        - - Ref: vpcDBSubnet1SubnetA8C3A9ED
          - Ref: vpcDBSubnet2SubnetDB39CD7E
          - Ref: vpcDBSubnet3Subnet54DAF0DD
    Export:
      Name: vpc-stack:vpcIsolatedSubnetIDs67369DC2
  vpcVpcIdE284377C:
    Value:
      Ref: vpcA2121C38
    Export:
      Name: vpc-stack:vpcVpcIdE284377C
  vpcVpnGatewayIdDCE6028A:
    Export:
      Name: vpc-stack:vpcVpnGatewayIdDCE6028A

[rix0rrr]

This is a bug that would have been caught early on if cdk.Output were to validate its properties.

I'm going to mark it as such; the fixer of this issue will have to deal with the special case of the VpcNetwork export (which should be unit tested already, so should throw once this fix is made).