Allow BuildkiteAgentTokenParameterStorePath to be an empty string as well

Author: James Healy

templates/aws-stack.yml

@@ -113,7 +113,7 @@ Parameters:
     Description: AWS SSM path to the Buildkite agent registration token (this takes precedence over BuildkiteAgentToken). Expects a leading slash ('/').
     Type: String
     Default: ""
-    AllowedPattern: "^/[a-zA-Z0-9_.-]+$"
+    AllowedPattern: "^$|^/[a-zA-Z0-9_.-]+$"
 
   BuildkiteAgentTokenParameterStoreKMSKey:
     Description: AWS KMS key ID used to encrypt the SSM parameter (if encrypted)