test(s3): add neither arn nor name are provided case (#25645)

Add test case for `parseBucketName` at `aws-s3/lib/util.ts`

Closes #<issue number here>.

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

Author: wafuwafu13

packages/aws-cdk-lib/aws-s3/test/util.test.ts

@@ -57,5 +57,10 @@ describe('utils', () => {
       const bucketArn = 'invalid-arn';
       expect(() => parseBucketName(stack, { bucketArn })).toThrow(/ARNs must/);
     });
+
+    test('undefined if neither arn nor name are provided', () => {
+      const stack = new cdk.Stack();
+      expect(parseBucketName(stack, {})).toBeUndefined();
+    });
   });
 });