feat(core): verify CfnOutput has a value and fix VPC export

Throw an error if `value` is undefined when defining a `CfnOutput`.

Fixes #2012 such that a VPC can be exported without a VPN gateway (there
is already unit test coverage).

packages/@aws-cdk/aws-ec2/lib/vpc.ts

@@ -483,9 +483,13 @@ export class VpcNetwork extends VpcNetworkBase {
     const priv = new ExportSubnetGroup(this, 'PrivateSubnetIDs', this.privateSubnets, SubnetType.Private, this.availabilityZones.length);
     const iso = new ExportSubnetGroup(this, 'IsolatedSubnetIDs', this.isolatedSubnets, SubnetType.Isolated, this.availabilityZones.length);
 
+    const vpnGatewayId = this.vpnGatewayId
+      ? new cdk.CfnOutput(this, 'VpnGatewayId', { value: this.vpnGatewayId }).makeImportValue().toString()
+      : undefined;
+
     return {
       vpcId: new cdk.CfnOutput(this, 'VpcId', { value: this.vpcId }).makeImportValue().toString(),
-      vpnGatewayId: new cdk.CfnOutput(this, 'VpnGatewayId', { value: this.vpnGatewayId }).makeImportValue().toString(),
+      vpnGatewayId,
       availabilityZones: this.availabilityZones,
       publicSubnetIds: pub.ids,
       publicSubnetNames: pub.names,

packages/@aws-cdk/cdk/lib/cfn-output.ts

@@ -78,6 +78,10 @@ export class CfnOutput extends CfnElement {
   constructor(scope: Construct, id: string, props: CfnOutputProps = {}) {
     super(scope, id);
 
+    if (props.value === undefined) {
+      throw new Error(`Missing value for CloudFormation output at path "${this.node.path}"`);
+    }
+
     this.description = props.description;
     this._value = props.value;
     this.condition = props.condition;