Set default value for tuf_main_page_suffix (#1252)

Both root-signing and root-signing-staging now have a index.html
(not a pretty one but the basic info is there).

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

terraform/gcp/modules/sigstore/variables.tf

@@ -105,7 +105,7 @@ variable "tuf_kms_location" {
 variable "tuf_main_page_suffix" {
   type        = string
   description = "path to tuf bucket's directory index when missing object is treated as potential directories"
-  default     = ""
+  default     = "index.html"
 }
 
 variable "ca_pool_name" {