Allow log_delivery_group input to be null

This change is created to fix a problem when trying to use this module as DRY code.

The log_delivery_group variable expects a map, which it will turn into the log delivery configuration.

There is a default map provided in this module, which creates the slow-log log group.

  1. Say you create a new root module calling this elasticache submodule. You want to use this same
     root module for all your environments, and just pass the root module different values to determine
     what it does in different environments.

  2. You use this new root module to deploy production with the default values here. default slow-log group is created.

  3. You deploy non-production, passing in a custom slow-log configuration. the custom slow-log is created.

  4. You go back to deploy production, but there is a problem now. Since your root module is always
     passing in some input variable to the elasticache submodule, the default value is always overridden.
     The only way to get production to work again is to duplicate the default variable from this elasticache
     module, into your own root module's variable, as its default.

Now you have to duplicate this module's variable defaults in your root module. Not very
DRY, and could cause issues later if the elasticache module changes and now your root module's defaults
don't apply anymore.

Add `nullable = false` to the elasticache module's input variable.
This prevents you from passing in a null value to the input.
If you pass in a null, Terraform will simply ignore the input as if it was never passed.

Terraform can't use a null here anyway; it dies saying it expects a map value, not a null.

So this doesn't break any existing functionality, but it does allow a root module to pass
a default `null` value for this input variable, which makes the elasticache module use
its default value.

Author: pwillis-oi

variables.tf

@@ -82,6 +82,7 @@ variable "ip_discovery" {
 
 variable "log_delivery_configuration" {
   description = "(Redis OSS or Valkey) Specifies the destination and format of Redis OSS/Valkey SLOWLOG or Redis OSS/Valkey Engine Log"
+  nullable    = false
   type        = any
   default = {
     slow-log = {