Allow empty environment name

Author: Ravi Pranjal

recipes/res/res_demo_env/assets/bi.yaml

@@ -36,7 +36,8 @@ Parameters:
   EnvironmentName:
     Description: (Optional) EnvironmentName must start with "res-"and should be less than or equal to 11 characters. Required to generate certificates.
     Type: String
-    AllowedPattern: ^res-[A-Za-z\-\_0-9]{0,7}$
+    AllowedPattern: ^$|^res-[A-Za-z\-\_0-9]{0,7}$
+
     Default: res-demo
   AdminPassword:
     Description: Provide the Active Directory Administrator Account Password Directly or Resource ARN to Secret Containing Password.