propagate allowed s3 bucket name pattern

Author: wleepang

src/templates/step-functions/sfn-aio.template.yaml

@@ -70,7 +70,7 @@ Parameters:
       (can contain lowercase letters, numbers, periods and hyphens).
       If left blank a unique bucket name will be generated.
     Type: String
-    AllowedPattern: "(?=^.{3,63}$)(?!^(\\d+\\.)+\\d+$)(^(([a-z0-9]|[a-z0-9][a-z0-9\\-]*[a-z0-9])\\.)*([a-z0-9]|[a-z0-9][a-z0-9\\-]*[a-z0-9])$)"
+    AllowedPattern: "((?=^.{3,63}$)(?!^(\\d+\\.)+\\d+$)(^(([a-z0-9]|[a-z0-9][a-z0-9\\-]*[a-z0-9])\\.)*([a-z0-9]|[a-z0-9][a-z0-9\\-]*[a-z0-9])$)|(^.{0}$))"
     ConstraintDescription: "Must respect S3 bucket naming conventions"
   ExistingBucket:
     Description: Does this bucket already exist?