fix(rds): allow setting backupRetentionPeriod=0 (#2875)

0 would get turned into `undefined` which would lead to the default value of 1.

Author: jogold

packages/@aws-cdk/aws-rds/lib/instance.ts

@@ -507,7 +507,7 @@ abstract class DatabaseInstanceNew extends DatabaseInstanceBase implements IData
     this.newCfnProps = {
       autoMinorVersionUpgrade: props.autoMinorVersionUpgrade,
       availabilityZone: props.multiAz ? undefined : props.availabilityZone,
-      backupRetentionPeriod: props.backupRetentionPeriod ? props.backupRetentionPeriod.toString() : undefined,
+      backupRetentionPeriod: props.backupRetentionPeriod !== undefined ? props.backupRetentionPeriod.toString() : undefined,
       copyTagsToSnapshot: props.copyTagsToSnapshot !== undefined ? props.copyTagsToSnapshot : true,
       dbInstanceClass: `db.${props.instanceClass}`,
       dbInstanceIdentifier: props.instanceIdentifier,

packages/@aws-cdk/aws-rds/test/test.instance.ts

@@ -406,6 +406,28 @@ export = {
       ]
     });
 
+    test.done();
+  },
+
+  'can deactivate backup'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const vpc = new ec2.Vpc(stack, 'VPC');
+
+    // WHEN
+    new rds.DatabaseInstance(stack, 'Instance', {
+      engine: rds.DatabaseInstanceEngine.Mysql,
+      instanceClass: new ec2.InstanceTypePair(ec2.InstanceClass.Burstable2, ec2.InstanceSize.Small),
+      masterUsername: 'admin',
+      vpc,
+      backupRetentionPeriod: 0,
+    });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::RDS::DBInstance', {
+      BackupRetentionPeriod: '0'
+    }));
+
     test.done();
   }
 };