fix(route53): cannot set TTL to 0 (aws#14060)

Closes aws#14039


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-route53/lib/record-set.ts

@@ -212,7 +212,7 @@ export class RecordSet extends Resource implements IRecordSet {
   constructor(scope: Construct, id: string, props: RecordSetProps) {
     super(scope, id);
 
-    const ttl = props.target.aliasTarget ? undefined : ((props.ttl && props.ttl.toSeconds()) || 1800).toString();
+    const ttl = props.target.aliasTarget ? undefined : ((props.ttl && props.ttl.toSeconds()) ?? 1800).toString();
 
     const recordSet = new CfnRecordSet(this, 'Resource', {
       hostedZoneId: props.zone.hostedZoneId,

packages/@aws-cdk/aws-route53/test/record-set.test.ts

@@ -68,6 +68,30 @@ nodeunitShim({
     test.done();
   },
 
+  'with ttl of 0'(test: Test) {
+    // GIVEN
+    const stack = new Stack();
+
+    const zone = new route53.HostedZone(stack, 'HostedZone', {
+      zoneName: 'myzone',
+    });
+
+    // WHEN
+    new route53.RecordSet(stack, 'Basic', {
+      zone,
+      recordName: 'aa',
+      recordType: route53.RecordType.CNAME,
+      target: route53.RecordTarget.fromValues('bbb'),
+      ttl: Duration.seconds(0),
+    });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::Route53::RecordSet', {
+      TTL: '0',
+    }));
+    test.done();
+  },
+
   'defaults to zone root'(test: Test) {
     // GIVEN
     const stack = new Stack();