fix(aws-elasticloadbalancingv2): cannot clear access logging bucket p…

…refix (#15149)

This PR makes the empty bucket prefix explicit in cloud formation resource instead of omitting the entry all together.  Cloud Formation won't let you remove the prefix once it has has been set unless it is explicitly set this way.  

closes #14044
----

*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-elasticloadbalancingv2/lib/shared/base-load-balancer.ts

@@ -233,6 +233,7 @@ export abstract class BaseLoadBalancer extends Resource {
    * environment-agnostic stacks. See https://docs.aws.amazon.com/cdk/latest/guide/environments.html
    */
   public logAccessLogs(bucket: s3.IBucket, prefix?: string) {
+    prefix = prefix || '';
     this.setAttribute('access_logs.s3.enabled', 'true');
     this.setAttribute('access_logs.s3.bucket', bucket.bucketName.toString());
     this.setAttribute('access_logs.s3.prefix', prefix);
@@ -247,7 +248,6 @@ export abstract class BaseLoadBalancer extends Resource {
       throw new Error(`Cannot enable access logging; don't know ELBv2 account for region ${region}`);
     }
 
-    prefix = prefix || '';
     bucket.grantPut(new iam.AccountPrincipal(account), `${(prefix ? prefix + '/' : '')}AWSLogs/${Stack.of(this).account}/*`);
 
     // make sure the bucket's policy is created before the ALB (see https://github.com/aws/aws-cdk/issues/1633)

packages/@aws-cdk/aws-elasticloadbalancingv2/test/alb/load-balancer.test.ts

@@ -149,6 +149,10 @@ describe('tests', () => {
           Key: 'access_logs.s3.bucket',
           Value: { Ref: 'AccessLoggingBucketA6D88F29' },
         },
+        {
+          Key: 'access_logs.s3.prefix',
+          Value: '',
+        },
       ),
     });
 

packages/@aws-cdk/aws-elasticloadbalancingv2/test/nlb/load-balancer.test.ts

@@ -96,6 +96,10 @@ describe('tests', () => {
           Key: 'access_logs.s3.bucket',
           Value: { Ref: 'AccessLoggingBucketA6D88F29' },
         },
+        {
+          Key: 'access_logs.s3.prefix',
+          Value: '',
+        },
       ),
     });
 
@@ -479,4 +483,3 @@ describe('tests', () => {
     });
   });
 });
-