Set allowed and cache methods as non nullable

Setting nullable to false ensures that the variable value will never be null within the module. If nullable is false and the variable has a default value, then Terraform uses the default when a module input argument is null.

variables.tf

@@ -196,12 +196,14 @@ variable "viewer_protocol_policy" {
 variable "allowed_methods" {
   type        = list(string)
   default     = ["DELETE", "GET", "HEAD", "OPTIONS", "PATCH", "POST", "PUT"]
+  nullable    = false
   description = "List of allowed methods (e.g. GET, PUT, POST, DELETE, HEAD) for AWS CloudFront"
 }
 
 variable "cached_methods" {
   type        = list(string)
   default     = ["GET", "HEAD"]
+  nullable    = false
   description = "List of cached methods (e.g. GET, PUT, POST, DELETE, HEAD)"
 }