fix(serverless): change cache_usage_limits default from {} to null

The serverless_cache_usage_limits variable had a default value of {}
(empty object), which caused the dynamic block to create an empty
cache_usage_limits block with no nested content. AWS rejects this
configuration during apply, resulting in:

  Error: Provider produced inconsistent result after apply
  .cache_usage_limits: block count changed from 1 to 0

Changing the default to null allows the dynamic block logic to
correctly skip creating the block when no limits are specified.

Author: oycyc

variables.tf

@@ -324,7 +324,7 @@ variable "serverless_user_group_id" {
 
 variable "serverless_cache_usage_limits" {
   type        = map(any)
-  default     = {}
+  default     = null
   description = "The usage limits for the serverless cache"
 }