fix(serverless): Prevent empty cache_usage_limits block causing provider inconsistency error, update default from {} to null

[oycyc]

Problem

The current default value for serverless_cache_usage_limits is {} (empty object). When this is used, the dynamic block creates an empty cache_usage_limits block with no nested content:

cache_usage_limits {
}

AWS/the provider expects either:

• No cache_usage_limits block at all, OR
• A cache_usage_limits block with at least one nested block (data_storage or ecpu_per_second)

This results in the following error during apply:

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

Changes

Changed the default value of serverless_cache_usage_limits from {} to null in variables.tf:

variable "serverless_cache_usage_limits" {
...
- default     = {}
+ default     = null
  description = "..."
}

This allows the existing dynamic block logic to correctly skip creating the block when no limits are specified:

dynamic "cache_usage_limits" {
  for_each = try([var.serverless_cache_usage_limits], [])
  # When var is null, try() returns [], so the block is not created

Issue

Closes #268

See #268 for more detail.

[oycyc]

/terratest

[oycyc]

Curious to see if this requires a MAJOR version? Since it changes the default functionality from {} to null. But again, it is both the equivalent of empty. BUT again the default was non-functional - it literally caused errors, so no one was successfully using it.

I'll hear from the community/other contributors to see what others think.